GIFT  OF 
Harry  East  Miller 


FIRST 


LESSONS  IN  PHYSICS. 


FOB    USE 


IN  THE  UPPER  GRADES  OF  OUR  COMMON 
SCHOOLS-.: 


• '  In  Nature  all  is  Motion ,  Life ,  and  Labor . ' '    Lesson  xxii . 


— BY — 


C .    L  .    H  O  T  Z  E  , 

Teacher  of  Natural  Sciences  in  the  Cleveland  High  School, 
Author  of  "  First  Lessons  in  Physiology ',"  etc. 


ST.  LOUIS: 
THE  CENTRAL  PUBLISHING  COMP'Y. 


C.    3, 

*', 
£ -75 


in  the  year  1871,  by 


In  ihe  Office  of  the  Librarian  of  Congress,  at  Washington 

Entered  according  to  Act  of  Congress,  in  the  year  1875,  by 

THE  CENTRAL  PUBLISHING  COMP'Y, 
In  the  Office  of  the  Librarian  of  Congress,  at  Washington. 


_, 
0 


PREFACE. 


The  conviction  that  an  elementary  knowledge  of  some 
important  instruments,  machines  and  physical  phenomena 
can  and  should  be  given  in  our  Common  Schools,  has  in- 
duced the  author  to  prepare  the  present  little  volume.  Its 
object  is  the  presentation  of  a  number  of  phenomena,  laws, 
and  applications  of  the  same,  specially  adapted  to  the  per- 
ceptive capacities  of  the  pupils  of  the  upper  grades. 

Inasmuch  as  the  demand  of  a  large  amount  of  time  might 
delay  the  introduction  of  physical  science  into  the  Common 
School,  the  book  has  been  so  prepared  as  to  secure  good 
results  in  the  minimum  of  time  ever  given  any  study  in  our 
schools,  viz. :  one  lesson  a  week. 

Each  of  the  thirty-nine  lessons  commences  with  a  fact 
familiar  to  the  child,  or  an  easy  little  experiment,  which 
serves  as  the  basis  for  the  development  of  a  natural  law. 
After  this  law,  comes  the  application  man  makes  of  it — such 
as  the  barometer,  thermometer,  pump  and  hydrostatic  press. 

Costly  apparatus  is  unnecessary.  A  pencil ,  a  marble ,  a 
piece  of  board,  of  India-rubber,  of  wire;  glass  tubes,  and 

M81995 


IV  PREFACE. 

other  objects  of  trifling  expense,  are  sufficient,  for  our  pur- 
poses even  preferable.  The  steam  engine  and  other  com- 
plicated machines  should  be  examined  when  in  actual  use 
at  the  workshop  or  other  places,  by  the  class  in  company 
with  the  teacher,  but  not  until  after  the  preparatory  lesson 
in  the  school-room. 

Like  all  instruction,  instruction  in  physics  should  proceed 
in  concentric  circles  from  the  near  to  the  remote.  The 
present  volume  may  be  considered  as  the  first  and  smallest 
of  those  circles.  Its  usefulness  in  the  highest  grade  of  our 
Common  Schools  has  been  shown  by  practical  experience ; 
the  author  has  written  it,  however,  with  a  view  of  intro- 
ducing it  into  the  second,  and  even  into  the  third. 

At  the  end  of  every  lesson,  articles  in  books  and  popular 
magazines  are  pointed  out,  where  the  pupil  may  find  inter- 
esting reading  matter ;  and  where,  while  thus  improving  his 
leisure  time,  he  may  collect  material  for  composition  exer- 
cises in  school. 

CLEVELAND,  O.,  Aprils,  1871 


Preface  to  the  Third  Edition. 


The  hearty  welcome  given  to  the  first  edition  of  this  work  undoubt- 
edly had  its  reason  in  the  long-felt  want  of  a  text-book  suitable  for  the 
thousands  of  girls  and  boys  whose  school  education  ends  in  the  com- 
mon school.  Among  the  many  things  there  learned,  there  are  few 
things  which  they  remember  to  greater  advantage  than  the  phenomena 
and  daily  applications  of  the  laws  of  gravitation,  the  pressure  of  air,  the 
lever,  the  pump,  the  steam-engine,  and  the  telegraph.  These  realities 
train  the  observing  powers,  instill  a  love  for  knowledge,  form  a  preven- 
tive against  habits  of  superficial  reasoning,  and  thus  tend  to  diminish 
explosions,  conflagrations,  and  other  calamities,  many  of  which  are 
caused  by  persons  ignorant  of  the  powers  of  nature.  The  merchant, 
the  laborer,  or  the  manufacturer  will  do  his  work  the  better  for  having 
had  his  senses  trained  in  observing  nature's  operations,  and  his  mind 
disciplined  by  scientific  thought.  It  may  safely  be  stated  that  this  view 
is  held  by  most  educators  in  this  country,  and  that  the  time  is  fast 
approaching  when  physical  science  will  no  longer  be  a  stranger  in  our 
common  schools. 

And  yet  there  are  a  few  followers  of  the  cramming-system,  who 
would  deny  the  right  of  nature  to  a  share  in  the  education  of  the  young; 
who  would  not  teach  about  the  things  themselves,  but  merely  their  names 
and  forms.  These  persons  consider  objective  instruction  in  the  lower 
grades  of  schools  as  simply  a  transient  concession  to  ephemeral  de- 
mands, although,  during  the  last  two  centuries,  such  men  as  Cowley, 
Milton,  Locke,  Rousseau,  Pestalozzi,  Whewell,  and  Macaulay  have 
advocated  it.  In  the  upper  grades  they  refuse  it  admission  altogether, 
notwithstanding  its  introduction  there  is  urgently  pressed  by  the  scientific 
men  of  all  countries,  by  the  entire  periodical  press,  and  the  most  prom- 
inent educators  of  the  world. 

These  few  opponents  to  progress  in  education  are  joined  by  a  still 
smaller  class  of  persons  who  are  not  adverse  to  the  introduction  of 


VI  PREFACE. 

physical  science  into  the  schools,  but  who  fear,  lest  the  appropriation  of 
time — one  lesson  a  week! — might  diminish  the  habitual  number  of  arith- 
metical examples,  geographical  names,  and  grammatical  rules,  and  there- 
by vitiate  the  results  of  the  annual  examinations.  So  some  people  en- 
tertain a  groundless  prejudice  against  the  acquisition  of  a  foreign 
language,  on  the  plea  that  the  child's  English  might  suffer.  Huxley, 
in  his  "Answers  to  Certain  Questions  by  the  Schools  Inquiry  Commis- 
sion," says :  "  Physics  lie  at  the  foundation  of  all  science ;  and  if  nothing 
else  were  tattght,  it  would  be  a  great  gain  to  have  the  youth  of  this 
country  soundly  instructed  in  the  laws  of  the  elementary  forces — gravi- 
tation, heat,  light,  and«so  forth."  An  English  Journal,  "  Nature,"  says : 
"  The  notion,  that  when  a  child  has  learned  to  read,  write,  and  cipher, 
he  is  educated,  must  be  eradi^zted.  These  are  at  best  but  means,  and  are 
only  the  instruments  by  which  education  is  conducted"  An  editorial  in 
the  "Scientific  American"  (January  14,  1871),  ends  with  the  following 
significant  words :  "  As  object  teaching  is  a  mere  handmaid  of  science — 
is  of  use  only  to  give  scientific  habits  of  "thought,  and  to  convey  a  know- 
ledge of  scientific  facts,  and  is  worthless  without  science,  the  pnblic  should 
see  that  its  introduction  into  our  schools  be  carried  on  under  the  advice 
of  scientific  experts,  who  shall  direct  what  is  best  to  be  taught,  and 
advise  with  the  adepts  in  teaching  how  such  knowledge  may  best  be 
imparted.  As  a  journal  having  the  interests  of  science  and  education  at 
heart,  desiring  to  see  science  soundly  popularized,  and  the  masses  made 
acquainted  with  its  technical  value,  we  make  this  suggestion,  and  further- 
more ask:  Is  there  any  man  of  scientific  attainments  in  the  present 
Board  of  Education  ?  Is  there  any  scientific  authority  upon  its  general 
staff?  " 

Physical  science  was  introduced  into  the  B  and  C  grammar  classes  ot 
this  city  last  September ;  the  pupils  have  now  been  using  First  Lessons 
in  Physics  for  several  months,  and  none  of  their  other  studies  have  been 
curtailed,  yet  the  average  of  the  monthly  examinations  does  not  suffer 
on  that  account,  and,  in  the  opinion  of  our  teachers,  it  never  will.  A 
peculiar  feature  connected  with  the  use  of  this  book — one  which  we  trust 
will  not  be  brought  forward  as  an  objection — is,  that  the  children  ask  a 
great  many  questions  more  or  less  to  the  point ;  and  that  they  find  no 
rest  until  they  have  received  a  satisfactory  answer,  either  from  the 
teacher's  experiments  or  their  own.  The  fact  is  truly  surprising,  that 


PREFACE.  VII 

the  pupils  of  the  C  grade  (sixth  school-year)  passed  a  very  fair  examina- 
tion a  few  days  ago,  on  questions  at  the  end  of  the  book  which  were  not 
found  too  easy  for  the  C  grade  of  the  High-school  (the  tenth  school- 
year),  This  shows  what  earnestness  may  accomplish;  and  we  have  but 
begun 

It  may  be  well  to  state  that  the  modern  technical  sense  of  a  word 
sometimes  conflicts  with  its  preconceived  English  meaning,  or  use;  and 
as  a  book  of  this  kind  demands  language  both  youthful  and  technical, 
the  author  may  be  excused  for  having  given  a  slightly  different  dress  to 
not  a  few  of  the  laws.  He  has  omitted  several  of  the  so-called  "pro- 
perties "  of  matter  which  are  very  puzzling  to  the  young;  and,  for  the 
sake  of  simplicity,  has  treated  the  somewhat  magic  "*' impenetrability  "  of 
air  as  elasticity  of  air.  The  independent  terms,  Force,  Motion,  and 
Heat,  are  better  understood  by  young  pupils  than  Expansive  Force, 
Moving  Force,  and  so  forth.  The  text  in  fine  print,  as  well  as  pages 
83,  84  and  1 20,  must  be  omitted  in  a  lesson  of  less  than  an  hour's  length. 
The  development  of  the  steam-engine  will  find  favor  from  those  appre- 
ciating the  historical  element  in  the  schooL  While  the  lessons  in  Optics 
may  claim  special  clearness  in  treatment,  those  in  Chemical  Electricity, 
being  very  difficult  for  young  learners,  will  need  forbearance.  ^  two- 
fluid  element  was  chosen,  because  it  may  be  seen  in  actual  use  at  the 
telegraph  office.  The  questions  in  fine  print  serve  for  reviews  and  ex- 
aminations, but  not  as  equivalents  for  experiments. 

Even  a  brief  perusal  of  the  volume  will  show  the  author's  intention 
not  to  cram  the  pupil  with  meaningless  facts,  to  be  forgotten  as  rapidly 
as  they  are  learned.  As  no  special  scientific  qualification  has  been  re- 
quired of  the  teacher  who,  to-morrow,  may  be  called  upon  to  impart 
scientific  instruction  to  her  class,  a  text-book  in  the  hand  of  the  pupil 
seems  for  the  present  a  necessity.  I  earnestly  hope  that  my  feeble  con- 
tribution to  so  great  a  cause  may  not  be  judged  by  its  shortcomings 
alone,  and  that  the  day  may  soon  come  when  physical  science  shall  form 
a  regular  branch  of  study  in  the  common  schooL 

CLEVELAND,  O.,  December  I,  1871. 


PEEFACE  TO  EEY1SED  EDITION, 


In  the  present  Edition,  Lessons  I,  VI,  IX,  XIII,  XX,  will 
be  found  materially  altered.  The  Lesson  on  the  Barometer  is 
entirely  new;  and  page  174  on  the  Thermometer  has  been 
added. 

Cleveland,  O.,  May  i,  1875. 


C.  KLEINR,  274  Eighth  Avenue,  New  York,  will  furnish  sets  of  inex- 
pensive Apparatus,  for  "  Hotze's  First  Lessons  in  Physics,"  or  any 
apparatus  required  for  the  demonstration  of  Physical  Science. 


CONTENTS. 


FORCE, 

— OF  ATTRACTION. 

PAGE 

LESS.    1. -Gravity 11 

"       2.— Gravity,  Specific— Floating  and  Sinking 14 

"       3.— Magnetic  Attraction 17 

"        4. — Electric  Attraction 20 

"       5.— Lightning.— Lightning  Rods 26 

"       6.— Cohesion 29 

' '       7.— Adhesion.— Capillary  Attraction 32 

"       8.— Review 36 

— OF  PRESSURE. 

LESS.    9.— Elasticity 39 

"      10.— Elasticity  of  Air 42 

"      11.— Pressure  of  Air 45 

"      12.— Barometer 48 

"      13 —Review 52 

"      14.— Inertia 54 

MOTION, 

—OF  MASSES. 

LESS.  15.— Inclined  Plane 56 

"      16.— Lever 59 

"      17. -Pendulum ".  63 

18.— Communicating  Vessels.— Hydraulic  Press..  67 

19.— Breathing.— The  Bellows 71 

20.— Common  Pump 74 

21.— Forcing  Pump.— Fire-Engine 77 

22.— Review 82 

— MOLECULAR. 

LESS.  23.— Sound 85 

"      24.— Evaporation,  Fog,  Clouds,  Rain,  Snow,  Hail, 

Dew,  Frost 88 

25.— Heat.— Conduction  of  Heat 92 

26.— Draught 96 

27.— Expansion  by  Heat.— Thermometer 99 

28.— Thermometer  Compared  with  Barometer 102 

29.  — Atmospheric  Engine 105 

30.—  Steam-Engine Ill 

31.— Review 11& 

32.— Light.— Its  Sources.— Direction 121 

33.  — Radiant  and  Specular  Reflection 121 

34.— Visible  Direction.— Refraction 127 

35  —Prisms.— Lenses 131 

36.  —Colors 135 

87 .  —Chemical  Electricity HO 

38.— Electro-Magnetic  Telegraph 143 

39.-Review 150 

QUESTIONS ™* 

APPENDIX * '  * 

INDKX.  .   17£> 


I,  Barometer;  2,  Microscope;  3,  Heron's  Fountain;  4,  Pump;  5,  Steam 
Engine ;  6,  Magnet ;   7,  Telescope ;  8,  Barometer ;  9,    Galvanic 
Battery;    10,   Hour-Glass;    n,    Clock;   12,  Thermom- 
eter ;   13,  Spirit  Lamp  ;   14,  Magnetic  Needle ; 
15,  Mirror;  16,  Balance;  17,  Weights  ; 
l8,    Pump. 


LESSON  I. 
GRAVITY:  «  ,% 


1.  EXPERIMENT.— A  stone  in  the  hand  does  not 
fall,  because  the  hand  supports  it.  But  if  we  drop 
the  stone,  it  falls,  and  will  continue  to  fall,  until  an 
obstacle,  such  as  the  floor  or  the  ground,  prevents 
it  from  falling  farther. 

Familiar  Facts.  —  Chalk,  pencils,  paper,  pens, 
and  India-rubber,  often  fall  from  the  desk  upon  the 
floor.  A  stone  thrown  into  a  pond  sinks  to  the 
bottom ;  a  sign-board  blown  off  by  the  storm  falls 
upon  the  side-walk;  rain,  snow,  and  hailstones, 
descend  to  us  from  the  clouds.  Heavy  rods  are 
attached  to  maps  and  curtains,  to  draw  them  down. 
In  clock-works  moved  by  weights,  the  weights  move 
in  a  downward  direction. 

Having  noticed  these  facts,  you  naturally  inquire : 
"  Why  is  it  that  all  bodies  near  the  earth  have  a 
tendency  to  fall  toward  it  ? "  As  every  state  and 
every  town  has  its  laws,  so  Nature  has  her  laws, 
which  all  bodies  must  obey.  All  the  facts  given 
above  may  be  comprised  under  the  law :  All  bodies 
fall,  if  unsupported ;  they  are  attracted  to  the 
earth.  The  force  which  attracts  them  is  called  the 
Force  of  Gravity. 


12 


FIRST   LESSONS   IN   PHYSICS. 


2.  EXPERIMENT.  —  This  stone  is 

not  supported  by  the  hand  (Fig.  1), 

but  it  is  suspended.     What  prevents 

it'f*o»  falling.^.  The  string.     When 

" 


"ftoje  a  little  to  one 
sides  £ia(L,tb.en  leX  it  gty  it  will  swing 
^afei^Siicl*  fttrthj  iafi'd  .fihally  come  to 
rest.  In  this  po'sitioii  the  string  in- 
dicates the  direction  in  which  an 
object  would  fall,  if  it  were  left  free 
to  fall.  This  direction  is  vertical. 
It  gives  rise  to  the  plumb-line  used 
by  carpenters  and  bricklayers. 

The  direction  in  which  bodies  fall 
when  tliey  are  moved  ~by  the  force  of 
gravity  alone,  is  vertical. 

3.  EXPERIMENT.  —  Place  a  large  book  upon  the 
hand  ;  the  hand  will  be  pressed  downward.  If  a 
small  book  be  taken,  the  downward  pressure  is 
much  less  ;  and  we  conclude  that  the  small  book 
has  less  weight  than  the  large  one. 

Familiar  Facts.  —  A  large  stone  presses  itself 
into  the  ground.  The  weight  of  a  heavy  wagon 
makes  deep  ruts  in  a  road.  When  ladies  buy  silk 
robes,  they  lift  the  article  on  their  hands  ;  you  will 
now  understand  why  this  is  done. 

The  pressure  which  bodies  exert  when  supported, 
or  the  pull  which  they  exert  wlien  suspended,  is 
called  weight. 


GRAVITY.  13 

4.  EXPERIMENT. — A  yard-stick  balanced  on  the 

edge  of  the  hand  has  equal  weight  on  each  side  ot 
the  support.  The  direction  of  the  rod  is  level,  or 
horizontal.  Now,  let  a  crayon  be  suspended  from 
each  end.  The  rod  will  still  be  horizontal,  provided 
both  crayons  have  like  weight.  If  a  number  of 
crayons  be  suspended  from  one  end  of  the  rod,  and 
a  weight  from  the  other,  we  have  a  crude  form  of 
the  scale,  or  balance. 

Allow  the  weight  of  fig.  1,  suspended  by  a  string, 
to  dip  in  water.  The  surface  of  the  water  is  hori- 
zontal ;  a  vertical  line  makes  a  right  angle  with 
any  horizontal.  If  a  line  makes  a  right  angle  with 
another,  both  lines  are  perpendicular  to  each  other. 

A  balance  is  an  instrument  for  weighing.  Pieces 
of  iron,  brass,  or  lead,  are  used  as  standards  ;  they 
are  called  the  weights.  There  are  different  kinds 
of  balances  ;  one  consists  of  a  delicately  poised  rod 
with  a  pan  suspended  from  each  end  for  holding  the 
weights ;  another,  also  of  a  delicately  poised  rod, 
but  with  the  pans  directly  over  its  ends.  Sub- 
stances placed  upon  the  former  exert  downward 
pull;  on  the  latter,  downward  pressure.  Then, 
there  is  the  steel-yard  and  the  spring-balance. 

Gravity  attracts  all  substances  alike.  Thus  a 
mass  of  feathers  weighing  a  pound  contains  as 
much  matter  as  a  mass  of  lead  weighing  a  pound ; 
a  pound  of  water  is  as  heavy  as  a  pound  of  iron. 

The  force  of  gravity  is  put  to  use  in  balances 
(the  spring-balance  excepted),  the  plumb-line,  clock- 
weights,  hour-glasses. 


14  FIKST   LESSONS   IN   PHYSICS. 


LESSON    II. 

SPECIFIC   GRAVITY — FLOATING   AND    SINKING 
OF    SOLIDS. 

5.  EXPERIMENT.  —  Take  two  ink-wells  of  like 
size  and  weight ;  fill  one  with  water,  the  other  with 
oil,  and  place  them  on  the  pans  of  a  balance.     The 
pan  with  the  water  will  be  found  to  be  depressed  ; 
evidently  the  water  has  more  weight  than  the  same 
bulk  of  oil.     Commonly  we  say  that  water  is  heavier 
than   oil ;   but  we  ought  to   say,  that  water   has 
greater  specific  weight  than  oil;    that  is,  a  given 
bulk  of  water  has  more  weight  than  the  same  balk 
of  oil ;  or,  water  is  denser  than  oil,  because  any  vol- 
ume of  water  has  a  greater  mass  than  the  same  vol- 
ume of  common  oil. 

Specific  Gravity  is  the  weight  of  a  substance 
compared  with  the  weight  of  a  like  bulk  of  some 
other  substance  taken  as  a  standard. 

6.  EXPERIMENT. — Now  first  pour  the  oil  into  a 
tumbler,  and  then  the  water.     The  latter  being  the 
heavier,  it  settles  to  the  bottom,  the  oil  rising  above 
it.     Thus  oil  floats  on  water,  because  it  has  less 
weight  than  the  same  bulk  of  water. 

Familiar  Facts. — Smoke  rises  high  into  the  air: 
balloons  ascend  into  the  clouds.  Each  is  lighter 
than  a  like  bulk  of  surrounding  air. 


SPECIFIC   GKAVITY. 

Fluids  of  different  specific  gravities  place  them- 
selves in  tlie  order  of  their  specific  gravities,  the 
heaviest  below,  the  lightest  above. 

7.  EXPERIMENT.  —  Drop  a  stone  into  a  tumbler 
filled  with  water ;  it  sinks.     A  piece  of  cork  would 
float.    Upon  one  pan  of  a  balance  place  a  tumbler 
filled  to  the  brim  with  water ;  upon  the  other  place 
as    many  weights   as   are    necessary  to  establish 
equilibrium.    Remove  the  tumbler  and  drop  a  stone 
into  it.     The  stone  will  sink  and  some  water  will 
run  over.      The  space  now  occupied  by  the  stone 
was  occupied  by  as  much  water  as  ran  over ;  this 
water  was  held  up  by  the  water  in  the   tumbler. 
Now,  if  the  stone  had  no  greater  weight  than  a  like 
bulk  of  water,  it  would  likewise  be  held  up  by  the 
water.    It  can  easily  be  shown  that  it  weighs  more, 
if  we  place  the  tumbler  containing  the  stone  on  the 
balance  again ;  the  tumbler  will  have  more  weight 
than  it  had  before. 

8.  EXPERIMENT. — An  empty  flask,  closed  with  a 
cork,  floats  on  water,  and  displaces  but  very  little 
water.     It  evidently  has  less  weight  than  a  like 
bulk  of  water.    It  might  float  even  if  it  contained  a 
few  small  objects,  or  if  half  filled  with  water;   but 

.  if  entirely  filled,  it  sinks. 

A  body  floats  on  water,  if  it  has  less  weight  than 
an  equal  bulk  of  water  ;  it  sinks,  if  it  has  more. 


16  FIKST  LESSONS   IN   PHYSICS. 

Familiar  Facts.  —  As  the  flask  just  mentioned 
is  enabled  to  float,  so  is  a  ship ;  but  if  it  springs  a 
leak  and  fills  with  water,  it  will  go  down.  The 
human  body  has  nearly  the  same  weight  as  a  like 
bulk  of  water,  and  will  float  provided  the  lungs  re- 
main filled  with  air. 

Persons  who  fall  into  the  water  and  cannot  swim, 
often  lose  their  lives  because,  when  they  first  sink, 
the  water  closes  their  mouth  and  nose,  preventing 
them  from  inhaling  air.  Frightened  by  this,  they 
lose  their  presence  of  mind,  and,  instead  of  holding 
their  breath,  exhale  the  air  from  their  lungs.  Thus 
they  diminish  their  volume,  and  are,  of  course,  more 
apt  to  sink.  Then  they  foolishly  extend  their  arms 
into  the  air ;  the  head  naturally  sinks,  and,  unless 
rescued,  they  are  drowned.  The  danger  is  greatly 
diminished  if  these  persons,  on  falling  into  the 
water,  quietly  take  in  a  full  breath  of  air  and  hold 
it ,  then  throw  the  head  back  so  that  only  the  mouth 
and  nose  remain  above  water.  The  object  of  this 
is  to  enable  the  expanded  chest  to  buoy  up  the 
heavier  portions  of  the  body,  so  as  to  keep  at  least 
the  nostrils  open  for  breathing. 

Applications  made  of  Specific  Gravity.  —  By 
means  of  specific  gravity  the  purity  of  liquids,  and 
the  value  of  many  other  substances,  may  be  ascer- 
tained. 


MAGNETIC   ATTRACTION.  17 


LESSON    III. 

MAGNETIC    ATTRACTION. 

9.  EXPERIMENT.  —  Suspend  an  iron  nail  by  a 

string.  The  direction  of  the  string  will  be  vertical 
(Lesson  1).  But  if  we  bring  a  magnet  near  the  nail, 
the  nail  will  swing  toward  the  magnet ;  the  more 
so,  the  nearer  the  magnet  is  brought  to  the  nail. 
When  near  enough,  the  nail  will  attach  itself  to  the 
magnet,  and,  if  separated  from  the  string,  will  not 
fall.  This  is  owing  to  Magnetic  Attraction. 

Reverse  the  last  experiment.  Suspend  the  mag- 
net by  a  string,  and  lay  the  nail  on  the  table. 
Holding  the  suspended  magnet  over  the  nail, 
steadily  bring  it  near  the  latter,  the  nail  will 
eventually  rise  and  adhere  to  the  magnet. 

The  preceding  shows  that  Magnets  and  unmag- 
netic  iron  attract  each  other. 

10.  EXPERIMENT.— If  iron  filings  be  placed  on  a 

piece  of  paper  or  glass,  &nd  a  magnet  be  held  be- 
neath, the  iron  will  also  be  attracted  by  the  magnet, 
and  the  little  particles  will  arrange  themselves  in 
curves  about  the  end  of  the  magnet.  Magnetic 
attraction,  like  attraction  of  gravity,  is  independent 
of  intervening  bodies  and  spaces. 
2 


18  FIRST   LESSONS   IN   PHYSICS. 

Let  the  magnet  be  placed  horizontally  in  the  iron 
filings  and  turned  round  several  times.  On  with- 
drawing it  we  find  that  it  is  covered  at  the  ends 
with  long  'threads  of  the  filings,  while  toward  the 
middle  they  become  shorter,  and  in  the  centre  of  the 
magnet  there  is  no  attraction  whatever. 

From  this  we  see  that  the  attractive  power  of  a 
magnet  resides  chiefly  at  the  ends  of  the  magnet. 

11.  EXPERIMENT.  —  The  ends  of  a  magnet  are 

called  its  poles.  Tie  a  string 
to  the  centre  of  a  bar  mag- 
net, and  suspend  it  from 
the  hand.  The  magnet  will 
vibrate  until  it  finally  takes 
a  certain  position,  which  it 
keeps.  If  disturbed,  it  will 
again  vibrate,  and  after 
FIG.  2.  many  vibrations,  resume 

the  same  position.  It  will  do  so  whether  in  the 
room  or  out  of  doors.  On  examining  its  direction, 
we  find  it  pointing  north  and  south.  That  end 
of  the  magnet  which  points  north  is  called  the  north 
pole  of  the  magnet,  that  pointing  south,  its  south 
pole. 

A  freely  suspended  bar  toagnet  points  north  with 
one  end  ;  south,  with  the  other. 

12.  EXPERIMENT.  —  Bring  the  north  pole  of  a 

magnet  near  the  north  pole  of  a  freely  suspended 
magnet,  this  north  pole  will  be  repelled.  Now,  if 
the  two  south  poles  are  brought  together,  they  also 


MAGNETIC   ATTRACTION.  19 

repel  each  other ;  but  when  the  north  pole  of  the 
one  is  near  the  south  pole  of  the  other,  there  will  be 
mutual  attraction  between  these  unlike  poles. 

Like  poles  repel,  unlike  poles  attract  each  other. 

Application. — The  most  important  application  of 
magnetic  attraction  is  the  Compass,  or  Magnetic 
Needle,  used  by  mariners  and  surveyors. 

A  steel  needle  is  easily  rendered  magnetic  by  means  of  a  magnet. 
Lay  a  needle  upon  the  table  and  hold  its  point  with  the  left  hand ;  with 
the  other  hold  the  magnet  vertically  with  one  pole  in  the  centre  of  the 
needle.  Then  pass  it  slowly  along  the  right-hand  part  of  the  needle,  rub- 
bing the  needle  in  the  direction  from  the  centre  to  the  eye.  When  arrived 
at  the  eye,  the  magnet  must  be  raised  from  the  needle  and  passed  through 
the  air  back  to  the  centre,  there  to  recommence  the  same  operation  with 
the  same  pole.  This  process  may  be  repeated  about  thirty  times.  After 
that,  the  magnet  is  reversed,  taken  into  the  left  hand,  and,'  while  the  right 
now  holds  the  needle,  placed  upon  the  centre  of  the  needle.  By  rubbing 
the  magnet  from  the  middle  of  the  needle  to  the  left  end,  returning  through 
the  air,  and  repeating  this  the  same  number  of  times  as  in  the  first  process, 
the  needle  becomes  a  true  magnet.  It  will  attract  iron,  and  be  attracted 
by  the  same ;  it  will  point  north  and  south,  if  suspended  at  the  middle  and 
left  to  move  freely. 

Magnets  often  have  the  form  of  a  horse-shoe,  so  that  the  poles  are 
brought  near  together  ;  this  more  than  doubles  their  supporting  capacity. 

Bead  "Magnetism"  in  Faraday's  "Six  Lectures  on  the  Various 
Forces  of  Matter."  New  York  :  Harper  &  Brothers. 

Read  "Terrestrial  Magnetism,"  in  Harper's  Monthly,  Vol.  I,  p.  651. 


20  FIRST   LESSONS   IN   PHYSICS. 

LESSON   IV. 

ELECTRIC   ATTRACTION. 

13.  EXPERIMENT. — Rub  a  piece  of  sealing  wax, 

a  bar  of  sulphur,  or  a  lamp-chimney,  with  a  piece 
of  flannel,  and  bring  it  near  light  bodies,  such  as 
tiny  bits  of  paper,  wafers,  or  small  feathers.  They 
will  adhere  to  the  sealing  wax,  sulphur  or  glass  ; 
these  have  become  electric,  and  have  now  the  power 
of  attracting  light  bodies. 

14.  EXPERIMENT.  —  Heat   a    piece  of  writing 
paper  over  a  stove  or  lamp.    Place  it  upon  a  table, 
rub  it  several  times  with  a  piece  of  India-rubber, 
and  then  bring  it  quickly  near  some  light  bodies ; 
it  will  attract  them. 

Both  experiments  teach  us  that  Friction  produces 
Electricity,  and  that  electric  bodies  may  exert  at- 
traction. 

The  ancient  Greeks  knew  that  if  amber  is  rubbed 
it  would  attract  light  bodies ;  and  as  the  Greek 
name  for  amber  is  Electron,  the  name  of  this  at- 
tractive force  is  Electricity. 

15.  EXPERIMENT.  —  If,  in  a  very  warm  room, 
where  there  are  but  few  persons,  and  where   the 
atmosphere  is  perfectly  dry,  we  bring  the  knuckle 
near  electrified  sulphur,  glass  or  paper,  we  may 
see    a    spark    pass    from    the    substance    to    the 


ELECTRIC    ATTRACTION.  21 

hand.1  At  the  same  time,  we  also  hear  a  crack- 
ling noise,  feel  a  slight  stinging  in  the  hand,  and 
may  notice  a  peculiar  odor. 

Familiar  Facts. — The  fur  of  a  cat  when  rubbed 
with  the  hand,  a  gutta-percha  comb  passed  through 
the  hair,  or  the  strong  friction  of  India-rubber 
bands  against  rapidly  moving  axles  or  wheels, 
will  produce  electric  currencs  more  or  less  per- 
ceptible according  to  circumstances.  If  an  electri- 
fied body  be  held  close  to  the  face,  a  peculiar  sen- 
sation is  felt,  as  though  the  face  were  being 
covered  with  a  cobweb  ;  this  is  on  account  of  the 
attraction  between  the  electric  object  and  the  fine 
hair  on  the  face,  which  causes  the  hair  to  move. 

But  what  has  become  of  the  electricity  that 
passed  from  the  sulphur,  or  glass,  to  the  knuckle 
while  emitting  a  spark  ?  If  it  had  remained  there, 
the  knuckle  would  certainly  attract  light  bodies; 
but  this  is  not  the  case.  Neither  the  knuckle  nor 
the  hand  is  now  electric.  The  electricity  spread 

i.  As  it  often  depends  upon  uncontrollable  circumstances  whether  a 
spark  can  be  obtained  by  such  simple  means,  the  following  contrivance 
has  been  suggested:  "Take  a  glass  tube  of  }4-inch  bore  and  a  little  over 
a  foot  long.  Then  take  an  iron  wire,  coil  it  spirally,  and  insert  it  into 
the  tube— the  windings  should  be  J^-inch  distant  from  each  other,  and 
must  rest  firmly  against  the  inner  surface  of  the  tube.  One  end  of  the 
wire  is  to  protrude  from  the  tube,  and  a  tin  ball  to  be  soldered  on  to  the 
protruding  end.  The  other  end  of  the  spiral  wire  should  not  extend 
farther  than  the  middle  of  the  tube,  in  order  that  about  six  inches  of  the 
tube  may  be  used  as  a  handle.  On  rubbing  the  tube,  a  spark  may  be 
obtained  from  the  tin  ball." 


22  FIRST   LESSONS   IN   PHYSICS. 

all  over  the  body  and  over  the  earth,  and  thus  it 
was  sensibly  lost.  If  we  bring  a  key  near  elec- 
trified sulphur  or  glass,  or  a  tin  ball  (see  foot  note 
p.  21),  the  electric  current  will  pass  over  to  the  key  ; 
but  the  electricity  which  the  key  receives  does  not 
stay  there ;  it  passes  into  the  hand,  and  thence 
through  the  body  to  the  ground.  This  shows  that 
metals  and  the  human  body  are  conductors  of 
electricity.  If  in  place  of  the  hand  or  the  key, 
we  take  sealing  wax,  silk,  or  glass,  or  India-rubber, 
these  objects  will  remain  electric  after  the  contact. 
In  other  words,  they  do  not  conduct  electricity. 
Hence  sealing  wax,  silk  and  glass  are  non-con- 
ductors of  electricity.  If  any  part  of  a  non-con- 
ductor of  electricity  is  electrified,  the  electricity 
confines  itself  to  the  part  that  has  been  electrified ; 
but  if  any  part  of  a  conductor  is  electrified,  the 
electricity  spreads  itself  instantly  over  the  whole 
surface. 

16.  EXPERIMENT. — Suspend  a  pith  ball,1  attached 

I.  "  Pith  balls  may  be  obtained  best  in  winter  from  young  elder-trees  of 
one  year's  growth.  The  stem  is  split  open  with  a  sharp  knife,  the  pith 
is  cut  into  small  pieces,  each  of  which  is  rolled  between  the  hands  into 
a  ball.  To  suspend  the  balls,  pierce  each  with  a  needle  carrying  a  silk 
or  linen  thread,  make  a  knot  on  the  opposite  side,  and  then  draw  the 
knot  tight  a  little  ways  into  the  ball.  The  linen  thread  should  be  very 
fine.  If  silk  thread  is  used,  care  must  be  taken  that  it  contain  no  metal- 
lic color,  as,  for  example,  Prussic  Blue,  and  that  no  cotton  thread  be 
inside,  as  cotton  is  a  good  conductor.  The  thread  to  which  the  little 
ball  is  attached  is  taken  from  three  to  five  inches  long  ;  one  with  a  ball 
at  each  end  should,  of  course,  have  double  the  length.  They  may  be 


ELECTRIC   ATTRACTION.  23 

to  a  silk  thread,  from  the 
hand  or  some  other  support 
(Pig.  3).  On  presenting  it  to 
an  electrified  bar  of  sealing 
wax,  it  will  Tbe  seen  that  the 
ball  is  attracted  by  the  seal- 
ing wax,  that  it  comes  in  con- 
tact with  the  same,  and  that, 
after  it  has  become  electric 
itself,  it  is  repelled.  If  we 
then  slowly  follow  it  with  the 
sealing  wax,  it  is  repelled 
still  farther.  The  repulsion 
between  the  two  bodies  con- 
tinues, until  the  aqueous  vapor  in  the  room,  or 
some  other  good  conductor,  or  the  contact  of  the 
hand,  deprives  the  ball  of  its  electricity. 

17.  EXPERIMENT.  —  In  a  similar  manner  sus- 
pend two  pith  balls  each  from  a  silk  thread.     On 
presenting  electrified  sealing  wax,  they  become 
electric  themselves  by  contact  with  it,  and  then 
repel  each  other.   They  hang  no  longer  vertically ; 
the  attracting  and  repelling  force  of  electricity 
may  overcome  gravity.      We  know  from  Lesson 
III,  that  magnetic  force  may  overcome  gravity. 

18.  EXPERIMENT.— Repeat  the  16th  Experiment 

with  a  single  pith  ball ;  after  it  becomes  electric, 

suspended  from  a  strong  wire  bent  at  right  angle,  which  may  be  inserted 
in  the  cork  of  a  bottle,  so  as  to  give  it  firm  support." 


24  FIRST   LESSONS   IN   PHYSICS. 

present  to  it  an  electrified  glass  rod  or  tube.1  The 
ball  will  be  immediately  attracted  by  the  elec- 
tricity of  the  glass. 

19.  EXPERIMENT. — Repeat  the  17th  Experi- 
ment, and  after  the  two  balls  are  separated  by 
repulsion,  present  electrified  glass  to  one  of  them. 
The  glass  will  attract  this  ball  and  impart  its 
electricity  to  it ;  after  which  the  ball  will  be  re- 
pelled from  the  glass  and  at  once  fly  to  the  other 
ball. 

When  the  two  balls  had  the  same  kind  of  elec- 
tricity, they  repelled  each  other ;  now  that  they 
possess  different  electricities — the  one  glass  elec- 
tricity, the  other  sealing  wax  or  resinous  elec- 
tricity— they  attract  each  other. 

20.  EXPERIMENT.  —  Bring  electrified    sealing 
wax,  or  gutta-percha,  near  one  of  the  two  balls, 
electrified  glass  near  the  other.     The  balls  will  at 
first  be  attracted  and  then  repelled,  when  they 
will  fly  toward  each  other  and   stay   together. 
This  is  easily  understood  if  we  remember  that 
one  ball  had  glass  electricity,  and  the  other  seal- 

I.  '« Glass  differs  greatly  with  respect  to  electrical  purposes.  Some 
varieties  are  good  conductors  of  electricity,  because  they  contain  metal. 
Hard  glass,  and  common  green  bottle  glass,  if  not  colored  with  metal, 
are  non-conductors,  and,  therefore,  well  adapted  for  that  purpose.  All 
kinds  of  glass,  however,  are  hygroscopic,  that  is,  they  draw  moisture 
from  the  atmosphere.  For  this  reason  thick  glass  rods  are  preferable 
to  glass  tubes.  Before  being  used,  both,  tubes  and  rods,  should  be 
slightly  heated,  and  should  be  rubbed  with  a  warm  cloth." 


ELECTRIC   ATTRACTION.  25 

ing  wax,  or,  as  it  is  called,  resinous  electricity. 
From  all  this  it  appears  that  there  are  two  kinds 
of  electricity —  Vitreous  or  Glass  Electricity,  and 
Resinous  Electricity.  The  former  is  also  called 
positive  electricity,  the  latter  negative  electricity. 
Like  electricities  repel  each  other ;  unlike  elec- 
tricities attract  each  other.  (For  a  similar  phe- 
nomenon see  the  preceding  lesson.) 

Historical. — The  sparks  obtained  by  the  rubbing  of  furs,  and  light- 
ning, with  its  companion,  thunder,  must  have  been  observed  by  the 
earliest  people  upon  the  earth.  Although  the  Greeks,  about  600  years 
before  the  Christian  era,  recorded  the  attracting  property  of  amber,  it 
was  not  before  the  beginning  of  the  iyth  century,  that  a  book  was 
published  by  Dr.  Gilbert,  an  Englishman,  who  mentions  many  other 
substances,  such  as  glass  and  sulphur,  as  having  the  same  property. 
This  author  stated  correctly  that  magnetism  attracted  as  well  as  repelled, 
but,  curiously  enough,  he  added  fliat  electricity  only  attracted. 

In  1670,  the  first  electric  machine  was  constructed  by  Otto  Guericke, 
burgomaster  of  Magdeburg,  the  inventor  of  the  air-pump.  He  also 
discovered  the  property  of  electric  repulsion.  He  excited  electricity  by 
means  of  sulphur  (brimstone)  exposed  to  friction. 

The  distinction  between  conductors  and  non-conductors  of  electricity 
was  discovered  by  Mr.  Stephen  Grey.  He  wished  to  electrify  a  cord 
suspended  by  linen  threads,  but  was  unsuccessful  because  the  electricity, 
when  entering  the  cord,  at  once  passed  over  to  the  threads.  The 
threads  thus  were  found  to  be  conductors  of  electricity.  Upon  the  sug- 
gestion of  a  friend  he  tried  silken  threads,  and  as  silk  is  a  non-con- 
ductor, the  experiment  then  met  with  the  desired  result. 

Du  Fay  distinguished  between  vitreous  and  resinous  electricity.  A 
number  of  other  scientists  afterward  improved  the  electric  machine,  and 
by  continuous  research  added  largely  to  the  progress  of  the  science.  But 
they  were  eclipsed  by  Dr.  Franklin  who  astonished  the  world  by  draw- 
ing electricity  from  the  clouds. 


26  FIRST  LESSONS  IN   PHYSICS. 


LESS01STY. 

LIGHTNING. — LIGHTNING    EODS. 

It  had  long  been  supposed  that  lightning  was  an 
electric  phenomenon,  but  it  was  not  until  1752  that, 
through  the  genius  of  our  countryman,  Benjamin 
Franklin,  all  doubts  were  removed.  Having  long 
been  thinking-  over  the  subject,  he  one  day  saw  a 
boy  fly  a  kite,  and  the  idea  at  once  struck  him  that 
he  must  make  one  himself  and  send  it  into  the 
clouds.  Accordingly  he  stretched  a  silk  handker- 
chief upon  two  sticks  in  the  form  of  a  cross,  on  the 
top  of  which  he  fastened  a  pointed  iron  wire.  This  he 
connected  with  the  hempen  string  holding  the  kite, 
and  upon  the  approach  of  a  thunder-storm  he  went 
out,  accompanied  only  by  his  little  son.  The 
hempen  string  was  attached  below  to  a  key,  and 
the  key  was  insulated  by  a  silk  string  which  Franklin 
held  in  his  hand.  The  clouds  were  passing  rapidly, 
but  without  any  apparent  effect  on  the  kite ;  and 
the  two  observers,  standing  below  and  watching  it 
with  great  anxiety,  were  about  to  abandon  the 
undertaking,  when  suddenly  the  fibres  of  the  string 
bristled  up,  and  a  crackling  noise  was  heard. 
Franklin  now  presented  his  knuckle  to  the  key, 


LIGHTNING — LIGHTNING   RODS.  27 

and  received  an  electric  spark,  which,  was  soon  fol- 
lowed by  an  abundance  of  sparks  as  the  string 
became  wet  with  the  falling  rain. 

Franklin's  experiment,  together  with  many  experi- 
ments by  scientific  men  in  Europe,  demonstrated 
beyond  a  doubt,  that  clouds  are  electric. 

Suppose  that  a  cloud  happens  to  contain  positive 
electricity, -it  will  then  call  forth  negative  electricity, 
either  from  some  neighboring  cloud,  or  from  the 
atmosphere,  or  from  the  earth.  And  if  its  electricity 
were  negative,  it  would  call  forth  positive  electricity. 
When  two  clouds,  or  a  cloud  and  the  earth,  are  suf- 
ficiently near  to  each  other,  their  electricities  unite. 
When  uniting,  one  of  them  leaps  over  the  space 
between  them.  This  passage  of  electricity  through 
the  air  produces  a  great  electric  spark  which  we 
call  Lightning. 

Familiar  Facts. — Electric  currents  may  pass 

from  one  cloud  to  another,  from  a  cloud  to  the  earth, 
or  also  from  the  earth  upward  to  the  clouds.  It  rarely 
happens  that  lightning  strikes — that  is,  strikes  ob- 
jects on  the  earth.  Tall  objects  made  of  good  con- 
ducting material  are  most  liable  to  be  struck, 
inasmuch  as  they  more  than  any  other  class  of 
bodies  attract  the  electricity  of  the  atmosphere  or 
clouds.  High  houses,  tall  steeples,  trees  or  chim- 
neys, therefore,  oflvr  a  good  passage  to  electricity. 
In  its  onward  course,  lightning  always  prefers  the 
best  conductors ;  thus  it  passes  along  the  spouting 


28  FIRST   LESSONS   IN   PHYSICS. 

of  houses,  along  water-pipes,  stove-pipes  and  iron 
pillars.  It  heats  metallic  objects ;  it  splits  trees 
into  fragments,  and  kills  living  beings  by  destroy- 
ing the  activity  of  their  nerves.  Lightning  some- 
times melts  small  metallic  objects. 

The  safest  place  during  a  thunder-storm  is  that 
part  of  a  room  not  too  near  the  fire  place,  stove, 
chandelier,  gas-pipe  or  bell-rope.  Why  is  it  unsafe 
to  seek  shelter  under  tall  trees,  or  in  the  entrance 
of  a  house  with  rain  pouring  down  over  it  ?  Know- 
ing that  electric  currents  follow  the  best  conductors, 
Franklin  invented  the  Lightning  Rod,  as  a  means 
to  direct  them  into  the~  ground.  It  consists  of  a 
copper  rod  with  a  metallic  point,  which  protrudes 
several  feet  above  the  roof,  in  order  that  on  the 
approach  of  a  current  the  metallic  point,  and  no 
part  of  the  building,  shall  be  struck.  The  rod  con- 
ducts the  electricity  into  the  earth  where  it  can  do 
no  harm.  But  the  connection  between  the  point  and 
the  earth  must  be  perfect  —  that  is,  the  rod  must 
not  be  put  in  direct  contact  with  the  wall,  else  the 
electric  current  may  pass  over  to  the  building  and 
produce  great  harm. 

The  purpose  of  lightning  rods,  then,  is,  1st,  to 
attract  electric  currents  ;  2d,  to  conduct  them  safely 
into  the  ground. 

Head  "  Thunderstorm, "  in  •'The  Earth  and  its  Wonders."  Cincin- 
nati :  Hitchcock  &  Walden. 


COHESION. 


LESSON    VI. 

COHESION. 

Familiar  Facts.  —  In  order  to   cut  meat,  to 

whittle  a  stick,  to  sharpen  pencils,  to  split  logs, 
to  saw  wood  or  to  plane  boards,  we  find  it  neces- 
sary to  use  instruments,  such  as  a  knife,  an  ax  or 
a  saw.  We  see  that  the  parts  of  a  solid  body  are 
not  easily  separated;  evidently  they  are  very 
close  together.  They  are  held  together  by  a  force 
which  we  call  Cohesion.  We  know  that  it  is 
difficult  to  break  a  piece  of  iron,  because  iron  has 
a  strong  cohesive  force  ;  yet  a  blow  with  a  poker 
sometimes  may  break  the  door  of  an  iron  st©ve. 
Rolled  or  hammered  iron  is  much  stronger  than 
cast-iron,  because,  by  the  process  of  rolling  or 
hammering,  its  particles  have  been  brought  nearer 
together,  and  hence  they  cohere  more  firmly. 
The  strength  of  our  tools  and  building-material 
depends  upon  cohesive  force.  The  supporting  ca- 
pacity of  wire  ropes,  iron  pillars,  or  steel  rods  is 
immense.  A  steel  bar  one  inch  square  will  support 
a  weight  of  forty  tons  or  more.  We  can  break 
wood  more  easily  than  iron,  because  it  has  less 
cohesive  force  ;  hence  the  particles  are  more  easily 
separated.  The  particles  of  water,  oil,  or  air,  are 


30  FIRST  LESSONS   IN   PHYSICS. 

separated  more  easily  still.  Place  the  hand  in 
water,  now  try  to  place  it  in  wood.  This  is  im- 
possible, for  the  particles  of  a  solid  body  are 
not  so  easily  separated  as  those  of  a  liquid. 
We  can  pour  water  from  a  pitcher  into  a  tumbler, 
and  oil  from  a  can  into  a  lamp.  The  cohesion  of 
liquids  is  well  exhibited  in  the  formation  of  water- 
drops  as  seen  when  water  is  gently  poured  from  a 
vial. 

To  break  a  solid  and  to  separate  the  particles  of 
a  liquid,  we  must  overcome  the  force  of  cohesion  of 
each. 

We  can  overcome  the  force  of  cohesion  of  a 
body  only  by  displacing  its  parts ;  we  do  not  in 
reality  penetrate  the  body.  Thus,  in  driving  a 
nail  into  a  board,  the  nail  crowds  parts  of  the  board 
aside.  One  body  can  not  occupy  the  space  of 
another  unless  the  other  body  be  first  removed ; 
that  is,  no  two  bodies  can  occupy  the  same  space  at 
the  same  time. 

Familiar  Facts.  —  When  a  little  child  breaks 
his  slate  he  tries  to  put  the  parts  together  again, 
but  he  quickly  perceives  that  they  will  not  remain 
together.  The  reason  of  it  is,  the  particles  on  the 
surface  of  the  edges  can  not  be  brought  so  near  to 
each  other  as  they  were  before  ;  that  is,  they  cohere 
no  longer.  A  broken  walking  cane,  although  the 
broken  parts  are  glued  together  again,  has  lost 
much  of  its  former  strength. 


COHESION.  31 

But  it  is  different  with  a  liquid.  Two  parts  of 
water  can  readily  be  made  to  form  one  mass  by 
pouring  them  together. 

Water  in  the  form  of  ice  has  more  cohesion  than 
in  the  liquid  state ;  in  the  form  of  steam,  no  cohe- 
sion whatever.  Solids  have  more  cohesion  than 
liquids ;  gases  have  none.  Because  by  heating  a 
solid  we  may  liquefy  it,  and  by  continuing  the  heat 
we  may  convert  it  into  gas,  heat  is  said  to  destroy 
cohesion. 

Familiar  Facts. — Although  solids  and  liquids 

cohere,  they  contain  a  great  number  of  little  holes, 
called  Pores.  These  may  be  of  different  size  in  the 
same  body,  and  may  be  visible  or  not.  The  pores 
of  our  sfcin  are  so  minute  that  they  can  not  be 
detected  without  a  magnifying  glass.  Every  square 
inch  of  the  skin  contains  about  1,000  pores.  Our 
health  depends  largely  upon  their  activity.1 

Solid  and  liquid  bodies  are  porous. 

Application. — (a.)  Of  Cohesion:  Beams  and  Pil- 
lars ;  Wire ;  Thread ;  Rope,  &c.,  &c  (b.)  Of  Porosity: 
The  Sponge ;  Blotting-Paper. 

I.  In  the  year  1661  the  Academy  of  Florence  proved  that  pores  exist 
even  in  gold.  A  thin  globe  of  gold  was  filled  with  water,  and  the  orifice 
carefully  closed.  A  violent  pressure  was  then  brought  to  bear  upon  it, 
and  the  result  was,  that  the  water  was  forced  through  the  pores  of  the 
gold,  and  stood  like  dew  upon  the  outer  surface  of  the  globe. 


32  FIRST   LESSONS   IN   PHYSIOS. 


LESSON    VII. 

ADHESION. — CAPILLARY  ATTRACTION. 

21.    EXPERIMENT.  —  Cut  two   leaden  bullets 

with  a  pen-knife  so  as  to  form  two  bright  surfaces, 
and  let  the  two  faces  be  pressed  against  each 
other  until  they  are  in  the  closest  contact ;  they 
will  be  found  to  adhere  firmly  to  each  other. 

Familiar  Facts. — The  same  takes  place,  if  a 
piece  of  India-rubber  be  cut  and  the  two  surfaces 
be  pressed  together.  Dealers  in  glass-ware  know 
that  when  mirrors  have  been  placed  together  with 
their  surfaces,  they  are  often  broken  in  the  at- 
tempt to  separate  them.  Between  solid  bodies, 
adhesion  takes  place  if  the  surfaces  are  highly 
polished;  that  is,  if  they  are  so  smooth  that  the 
parts  of  one  surface  closely  approach  those  of 
the  other.  If  not  highly  polished,  the  surfaces 
will  not  adhere,  as  two  bricks  laid  together.  Nor 
will  adhesion  take  place,  if  thin  paper  is  placed 
between  the  two  polished  surfaces. 

As  a  general  thing,  bodies  which  we  wish  to 
adhere  to  one  another,  are  not  very  smooth. 
Owing  to  the  unevenness  on  their  surface,  many 
of  the  parts  of  one  surface  are  prevented  from 
coming  in  close  contact  with  those  of  the  other ; 


ADHESION. — CAPILLARY    ATTRACTION.  33 

in  this  case  there  can  be  no  adhesion.  What  may 
be  done,  then,  in  order  to  make  two  rough  surfaces 
adhere?  Simply  put  a  liquid  body  between  the 
two  to  fill  out  the  unevenness. 

22.   EXPERIMENT.  —  Put  two  moistened  glass 

plates  together,  and  it  will  require  some  effort  to 
separate  them.  The  same  may  be  found  if  two 
boards  are  placed  together  with  water  between 
them.  In  both  cases,  however,  the  pressure  of  air 
against  the  exterior  surfaces  greatly  helps  to  hold 
them  together. 

Why  do  postage  stamps  adhere  to  envelopes  ? 
Because  there  is  cohesion  among  the  particles  of 
the  mucilage,  and  adhesion  between  the  mucilage 
and  each  paper  surface. 

Why  does  the  hand  become  wet  when  immersed 
in  water  ?  Why  does  it  remain  dry  when  drawn 
out  from  mercury  ?  Because,  in  the  first  case,  the 
adhesive  force  between  the  water  and  hand  is 
stronger  than  the  cohesive  force  of  the  water ;  in 
the  other  case,  the  cohesive  force  of  the  mercury  is 
stronger  than  the  adhesive  force  between  it  and  the 
hand.  Thus,  when  the  hand  is  placed  in  water,  a 
struggle  takes  place,  as  it  were,  between  the  parti- 
cles of  water  next  to  it  and  those  farther  away. 
The  adhesive  force  of  the  former  being  stronger 
than  their  cohesive  force,  they  cling  to  the  hand. 

8 


34 


FIEST   LESSORS   IN   PHYSICS. 


Adhesion  is  the  attraction  between  the  surfaces 
of  bodies  in  contact  with  each  other. 

Application. — All  gilding,  painting,  whitewash- 
ing, cementing,  varnishing,  gluing,  writing,  solder- 
ing, coating  of  looking-glasses,  plating,  &c.,  &c. 
Soot  adheres  to  the  chimney  ;  dust  to  the  ceiling ; 
chalk,  or  fresh  paint,  to  one's  clothing. 

23.  EXPERIMENT.— Immerse  a  clean  glass  plate 

partly  in  water,  some 
of  the  water  will  be 
seen  to  rise  on  both 
sides  of  the  plate. 
Evidently  the  adhe- 
sive force  between 
the  glass  and  the 
water  is  greater  than 
the  cohesive  force  of 
the  water.  Were  it  FIG.4. 

not  so,  the  water  would  not  rise.  Now  immerse 
another  glass  plate  near  the  first  and  not  parallel 
to  it  (Fig.  4).  Water  will  rise  between  them,  and 
the  form  of  its  surface  will  be  concave.  The 
nearer  the  glass  plates  are  brought  to  each  other 
the  higher  will  the  water  rise  between  them.  This 
is  natural,  for  the  quantity  of  water  between  them 
is  in  this  case  very  small;  and  the  cohesive  force 
of  the  water,  therefore,  easily  overcome  by  the 
adhesive  force.  If  a  glass  tube  be  immersed  (see 


ADHESION — CAPILLARY    ATTRACTION.  35 

Fig.  5)  the  water  will  rise  still  higher, 
because  here  is  a  small  quantity  of 
water,  surrounded  on  all  sides  by  glass, 
and  the  force  of  adhesion  is,  therefore, 
comparatively  of  greater  effect.1 

Capillary  tubes  (from  the  Latin,  capil- 
lus,  a  hair),  are  open  tubes  having  a 
very  narrow  bore.  When  such  a  tube 
comes  in  contact  with  a  liquid  which  is  FIG  6. 
capable  of  moistening  it,  the  liquid  is  compelled  to 
rise  in  it.  The  finer  the  bore  of  the  tube,  the  higher 
will  the  liquid  ascend.  In  a  tube  TJ^  of  an  inch  in 
diameter  water  will  rise  over  five  inches. 

Capillary  attraction  is  the  result  of  adhesion 
operating  between  solid  and  liquid  bodies. 

Application.— Sponge,  blotting-paper.  Eggs  and 
meat  may  be  kept  fresh  in  sand  or  pulverized  char- 
coal, these  two  substances  containing  .capillary 
tubes  which  absorb  any  moisture  that  would  other- 
wise affect  the  eggs  or  the  meat.  Lampwicks  also 
contain  capillary  tubes ;  they  suck  up  the  oil  in  the 
lamp.  Sugar  also  has  capillary  tubes.  Grease 
spots  on  the  floor  may  be  removed  by  laying  earth 
upon  them.  Our  clothes  become  wet  from  the  rain. 
In  short,  almost  everything  about  us  is  filled  with 
fine  capillary  tubes. 

i  But  the  reverse  of  all  these  phenomena  takes  place — that  is,  water  is 
always  depressed  about  glass  surfaces,  if  these  are  greased.  Grease  has 
no  attraction  for  water ;  the  water,  consequently,  is  left  free  to  obey  its 
cohesive  force,  and  falls  below  the  level  of  the  liquid  surrounding  the  tube. 
In  this  case  we  have  capillary  depression. 


36  FIEST   LESSONS   IN   PHYSICS. 

LESSON    VIII. 

REVIEW. 

LESSON  i. — 

All  bodies  are  attracted  to  the  earth.  The 
force  which  attracts  them  is  called  the  Force 
of  Gravity. 

2.  All  bodies  fall  if  unsupported;  if  supported, 

they  press  upon  their  support ;  if  suspended, 
they  pull  in  the  direction  in  which  they  would 
fall  if  left  free  to  do  so. 

3.  The  direction  in  which,  a  body  falls,  if  it  if 

moved  by  gravity  alone,  is  vertical. 

4.  T Tie  pressure  of  bodies  upon  their  support,  or 

their  pull  when  suspended,  is  called  Weight. 

5.  A  pound  is  a  weight,    indicating  a  certain 

amount  of  that  pressure  taken  as  a  standard. 

LESSON  ii. — 

6.  The  specific  gravity  of    a  substance   is    its 

weight,  compared  with  the  weight  of  a  like 
bulk  of  some  other  substance  taken  as  a 
standard.  When  we  say,  mercury  has  a  spe- 
cific gravity  of  13.6,  we  mean  that  any  bulk 
of  mercury  has  13.6  times  as  much  weight  as 
a  like  bulk  of  water. 


EBVIEW.  37 

7.  Fluids  of  different  specific  gravities,  when 

brought  together,  place  themselves  in  the 
order  of  their  specific  gravities,  the  heaviest 
below. 

8.  A  body  which  is  lighter  than  a  quantity  of 

water  of  equal  bulk,  floats  on  water;  one 
which,  is  heavier,  sinks. 

LESSON  in. — 

9.  The  attraction  between  magnets  and  iron  is 

called  Magnetic  Attraction.  The  magnetic 
attraction  of  the  earth  causes  the  magnetic 
needle,  or  any  magnet  freely  suspended,  to 
point  north  and  south. 

LESSON  iv. 

10.  The  attraction  of  electrified  bodies  is  called 
Electric  Attraction. 

LESSON  vi. — 

11.  The  parts  of  a  body  are  kept  together  by 
their  mutual  attraction.     The  attraction  be- 
tween the  parts  of  the  same  body  is  called 
Cohesion. 

12.  In  order  to  separate  tlie  parts  of  a  body,  we 
must    overcome   its   cohesion.      Solids    have 
more  cohesion  than  liquids.     Gaseous  bodies 
have  no  cohesion  whatever. 


38  FIRST   LESSONS   IN   PHYSIOS. 

LESSON  vii. — 

13  The  attraction  between  the  surfaces  of  bodies 
in  contact,  is  called  Adhesion. 

14.  The  adhesion  between  solids  and  liquids  is 
often  called  Capillary  Attraction. 

15.  Gravity,  Magnetism,  Electricity,   Cohesion, 
and  Adhesion,  are  forces  of  attraction      The 
last  two  are  called  Molecular  Forces,  because 
they  bind  molecules1  together. 

16.  The  first  three — Gravity,  Magnetism  and  Elec- 
tricity— act  through  great  distances  ;    adhe- 
sion and  cohesion  only  at  an  insensible  dis- 
tance. 

17.  Instead  of  magnetic  and  electric  attraction, 
we  may  witness  magnetic  and  electric  Repul- 
sion, while  gravity,  cohesion   and  adhesion, 
however,  exert  only  attraction. 

Questions. — What  natural  force  is  applied  in  the 
balance — the  compass — the  lightning  rod — 
suspension  bridges — blotting  paper  ? 

I.  "A  molecule  is  the  smallest  particle  of  matter  into  which  a  body 
can  be  divided  without  losing  its  identity."  Thus,  the  smallest  particles 
of  bread  or  of  salt,  which  are  still  bread  or  salt,  respectively,  are  mole- 
cules of  bread  or  of  salt. 


ELASTICITY.  39 


LESSON    IX. 

ELASTICITY. 

23.  EXPEKIMENT.  —  Take  an  ivory  ball ;  press 

it  with  your  hand  upon  a  slab  of  marble  that  has 
been  blackened  over  a  lamp.  The  ball  will  show  a 
black  spot  about  as  large  as  a  pin's  head.  Now  lay 
the  slab  upon  the  floor,  stand  on  the  table,  and  let 
the  ball  drop  upon  the  slab  from  a  considerable 
height  The  ball  will  then  have  a  black  spot 
much  larger  than  before.  Although  of  a  hard  sub- 
stance, the  ball  is  flattened  to  that  extent  when  it 
strikes  the  slab,  and  in  resuming  its  former  shape, 
it  rebounds. 

If  the  string  of  a  cross-bow,  be  drawn,  and  it  is 
then  let  go,  the  arrow  placed  before  it  flies  off 
with  astonishing  rapidity.  "  How  is  it,"  may  we 
ask,  "that  a  string  can  obtain  such  great  force?" 
If  a  piece  of  india-rubber  be  doubled  between  the 
fingers,  when  the  pressure  is  removed,  it  will  be- 
come straight  again.  After  pressing  a  steel  pen 
gently  against  the  thumb  nail  to  try  its  writing 
qualities,  it  recovers  its  former  shape. 

Springs,  ivory,  or  india-rubber,  and  many  other  bodies, 
have  the  property  of  recovering  their  former  figure  when 
the  force  which  distorts  them  ceases  to  act,  provided  the 
force  be  not  too  great. 


40  FIKST   LESSONS   IN   PHYSICS. 

This  property  is  called  Elasticity ;  and  all  such 
bodies  show  readily  that  they  are  elastic.  All 
bodies  are  more  or  less  elastic;  some  substances, 
however,  such  as  lead  or  clay,  do  not  readily  ex- 
hibit their  elasticity,  and  were  formerly  called 
inelastic. 

Application  of  Elasticity.  —  1.  To  produce  mo- 
tion: Watch-springs  ;  springs  in  watch  cases,  boxes, 
and  carriage-lanterns ;  the  ballista  of  the  ancients ; 
the  cross-bow;  locks,  and  triggers.  2.  To  coun- 
teract concussion :  Wagon- springs  ;  packing  glass 
ware  in  hay  or  straw;  springs  in  mattresses,  sofas, 
chairs  and  etui- cases.  3.  To  cause  close  contact  or 
pressure :  Springs  in  pocket-inkstands ;  printers' 
cylinders;  some  kinds  of  pen-holders.  4.  For 
weighing:  Spring- balances. 

Hardness  —  Softness.  —  An  ivory  ball  is  elastic 
and  hard ;  an  India-rubber  ball  is  elastic  and  soft. 
If  we  take  a  glass  rod  and  try  to  bend  it  as  much 
as  a  steel  rod  can  be  bent,  it  will  break.  The  steel 
rod  is  hard  and  elastic,  while  the  glass  rod  is  hard 
but  much  less  elastic.  Moist  clay  is  soft  but  has 
very  little  elasticity. 

All  this  shows  that  no  intimate  connection  exists 
between  elasticity,  and  hardness  or  softness.  A  body 
is  hard,  when  a  great  force  is  required  to  scratch  it, 
or  to  displace  its  particles ;  a  body  is  soft,  when 
its  particles  can  be  displaced  by  a  slight  force. 


ELASTICITY.  41 

Brittleness.  —  To  break  a  steel  rod  requires  a  great 
force  ;  while,  on  the  other  hand,  a  pane  of  glass  can 
be  broken  by  a  slight  blow.  Every  substance  can 
be  broken,  but  the  degree  of  force  required  is  not 
the  same  for  each.  Glass  or  chalk  is  brittle,  that 
is,  easily  broken.  The  property  of  being  easily 
fractured,  is  called  Brittleness. 

Ductility.  —  Instead  of  breaking  the  steel  rod,  let 
us  endeavor  to  stretch  it  ;  we  find  that  only  by  a' 
very  great  force  can  it  be  drawn  out  into  a  long 
wire.  A  narrow  glass  tube  can  be  drawn  out  very 
thin  by  the  heat  of  a  flame.  Bodies  which  can  be 
drawn  out  into  a  thread  or  wire  are  ductile  ;  this 
property  is  called  ductility. 

Malleability.  —  Many  substances,  such  as  iron, 
copper,  brass,  gold,  or  silver,  besides  being  ductile, 
can  -be  hammered  into  sheets  ;  hence  they  are 
called  malleable.  Some  substances,  as  lead  for 
example,  are  malleable  without  being  ductile.  Gold, 
the  most  malleable  metal,  has  been  hammered  out 
into  leaves  each  onl  sWoTJTj  of  an  inch  thick. 


From  this  lesson  we  may  learn  that  when  the 
force  which  displaces  the  parts  of  a  body  is  within 
certain  limits,  the  body  may  exhibit  elasticity  ;  if 
beyond  certain  limits,  it  exhibits  the  properties  of 
brittleness,  ductility  or  malleability. 


42  FIKST   LESSONS   IN    PHYSICS. 

LESSON    X. 

ELASTICITY   OF   AIR. 

24.  EXPERIMENT.— If  we  immerse  an  inverted 
tumbler  perpendicularly  in  water,  only  a  very  little 
of  the  water  will  enter  the  tumbler,  and,  of  course, 
the  air  in  the  tumbler  is  compressed.    If  the  ves- 
sel is  pressed  down  still  farther, 

a  little  more  water  enters  it,  but 

it  will  never  be  entirely  filled 

with  water,  because  it  contains 

air.     A  cork  previously  placed 

in  the  tumbler,  will  show  the 

position  of   the   water-level  in-  FIG  6< 

side.   (See  Fig.  6.)     Air  maintains  its  place  like 

every  other  body ;  hence  it  shuts  out  the  water 

almost  completely.    But  if  you  withdraw  the  hand 

which  presses  the  tumbler  down,  the  tumbler  will 

instantly  rise.  The  air  in  the  glass  was  compressed, 

and  recovered  its  previous  space,  because  air  is 

elastic. 

25.  EXEPERIMENT.— If  a  glass  funnel  be  im- 
mersed instead  of  a  tumbler,  and  if  inverted  with 
the  mouth  downward,  the  upper  end  being  closed 
with  the  thumb,  the  air  in  the  funnel  is  compressed. 
As  the  thumb  is  removed,  however,  water  rushes 
into  the  funnel,  because,  in  this  case,  the  air  can 
pass  out. 


ELASTICITY   OF   AIR.  43 

26.  EXPERIMENT.  —  Cement  a  funnel  into  the 
neck  of  a   small   bottle,  and  pour  water    into  it. 
Only  a  little  water  will  enter  at  first ;   but,  sub- 
sequently, not  a   drop  will  get  in,  as  there  is  no 
escape   for  the  air.      For,  as  the  water  is  poured 
into  the  funnel,  it  forces  the  air  in  the  tube  of  the 
funnel  into  the  bottle.     The  air  in  the  bottle  has 
now  no  outlet,  and,   consequently,  no  water  can 
enter. 

27.  EXPERIMENT. — A  very  good  illustration  of 

the  expansive  force  of  air  may  be  obtained  by  a 
"  Hero's  Fountain."  Take  a  cork  which 
fits  into  a  bottle,  and  perforate  it  with  a 
round  file.  •  The  hole  should  be  made  so  as 
to  admit  with  difficulty  a  glass  tube,  which 
is  now  pushed  through  the  cork.  The  tube 
should  have  a  very  fine  opening  above. 
This  being  done,  fill  the  bottle  about  half 
with  water  and  close  it  with  the  cork.  Then 
drive  the  glass  tube  farther  down,  until  it 
nearly  reaches  the  bottom  of  the  bottle.  The  bottle 
now  contains  air  in  its  npper  part  and  water  in  its 
lower.  On  blowing  more  air  into  the  tube,  the  air 
will  ascend  through  the  water  (Lesson  II)  and  col- 
lect in  the  space  above.  But  now  the  air  over  the 
water  is  compressed,  and  its  expansive  power 
forces  the  water  upward  through  the  tube.  The 
inventor  of  this  little  apparatus  was  Hero,  a  phil- 
osopher, who  died  in  Alexandria,  over  two  thous- 
and years  ago. 


44  FIRST   LESSONS   IN   PHYSICS. 

Air  is  an  elastic  T)ody  ;  the  more  we  compress  it,  the 
greater  is  its  expansive  force. 

A  useful  application  of  this  property  of  air  is  the 
air  chamber,  used  in  connection  with  pumps.  (Comp. 
Less.  21,  p.  78.)  The  Diving-bell  may  also  be  con- 
sidered an  application  of  this  force. 

'  Another  application  is  the  pop-gun.  A  piston 
moves  air-tight  in  the  tube  of  the  pop-gun.  Let  it 
be  at  one  end  of  the  tube ;  then  insert,  air-tight,  a 
stopper  into  the  other  end  of  the  tube  and  com- 
mence pushing  down  the  rod  ;  the  air  inside  is  now 
compressed,  it  has  the  tendency  to  expand  again ; 
but  its  force  is  not  great  enough,  as  yet,  to  drive 
out  the  stopper.  If  the  rod  is  pushed  in  farther, 
the  air  is  compressed  still  more,  and  the  stopper  is 
finally  expelled  with  a  loud  report. 

Another  application  is  the  blow-gun.  It  consists 
of  a  long,  smooth  wooden  tube,  into  which  is  fitted 
a  sharp  nail,  around  whose  head  shreds  of  cotton 
are  tied.  This  nail  is  inserted,  and  by  blowing  into 
the  tube  at  the  same  end,  a  great  quantity  of  air  is 
forced  in,  compressing  the  air  inside  ;  this  causes 
the  nail  to  move  forward.  On  blowing  more 
strongly,  the  air  is  compressed  more,  and  its 
expansive  force,  therefore,  greatly  increased.  The 
nail  is  then  expelled  from  the  tube,  and  its  speed 
will  be  in  proportion  to  the  force  with  which  you 
have  blown  into  the  tube. 


PRESSURE   OF    AIR. 


LESSON    XI. 

PRESSURE    OF    AIR. 

28.  EXPERIMENT. — A  tumbler  filled  with  water 
to  the  brim,  with  a  piece  of  paper 

placed  over  it,  is  inverted.  (See 
Fig.  8.)  The  hand  on  the  paper, 
after  pressing  the  latter  firmly 
against  the  tumbler,  is  removed, 
but  the  water  does  not  flow  out. 
How  can  this  be  accounted  for? 
Notice  that  the  tumbler  contains  FIG.  e. 

no  air ;  it  is  entirely  filled  with  water.  The  air 
evidently  presses  upward  against  the  paper.  It 
is  this  upward  pressure  of  the  air,  which  supports 
the  paper*(and  the  paper  supports  the  water),  or  else 
the  air  would  force  its  way  into  the  water,  by  rush- 
ing up  along  a  part  of  the  inner  side  of  the  tum- 
bler, leaving  the  water  to  fall  down  on  the  opposite 
part. 

29.  EXPERIMENT.  —  Immerse  a  tumbler,  hori- 
zontally, in  a  bowl  of  water,  and  press  it  down 
gradually.    It  will  fill  with  water,  and  afterward 
be  entirely  below  the  surface  of  the  liquid.     Now 
turn   it  bottom  upward,   and  without,   however, 
raising  its  mouth  above  the  surface,  lift  it  as  high 
as  possible.    The  whole  tumbler  is  still  filled  with 
water,  and  will  remain  filled,  although  the  water 


46 


FIRST   LESSONS   IN   PHYSICS. 


in  two  communicating  vessels  ought  to  have  the 
same  height  (Lesson  XVIII).  The  tumbler  contains 
no  air,  while  a  large  amount  of  air  is  over  the  re- 
maining water,  pressing  downward  upon  the  water. 
It  is  this  downward  pressure  of  air  which  supports 
the  column  of  water  in  the  tumbler. 

30.  EXPERIMENT.— Let  a  vessel  be  filled  with 
water;  then  take  a  narrow  glass 
tube,  open  at  both  ends,  and  im- 
merse it  perpendicularly  in  the  ves- 
sel. The  tube  will  be  partly  filled  with 
water;  if  taken  out,  the  water  will 
flow  through  the  tube  and  fall,  be- 
cause attracted  to  the  earth.  Place 
the  tube  again  in  the  vessel  with 
water,  but  on  slowly  removing  it,  be 
sure  that  you  keep  the  upper  open- 
ing closed  with  the  thumb.*  No  water 
will  flow  from  the  tube,  because  air 
presses  against  the  lower  opening 
and  thus  supports  the  column  of 
water  in  the  tube.  On  removing  the 
thumb  the  water  will  flow  out,  be- 
cause, in  that  case,  the  air  presses  as 
strongly  above  as  it  does  below  ;  the 
water,  consequently,  obeys  the  force 
of  gravity  and  falls.  Now  fill  the 
glass  tube  again  ;  hold  it  obliquely, 
until  it  is  in  a  horizontal  position  : 
FIG  9-  the  water  will  still  remain  in  the  tube. 


PRESSURE   OF   AIR.  47 

This  shows  that  air  presses  not  only  upward ', 
downward,  and  laterally,  but  in  all  directions. 

In  the  preceding  two  experiments,  a  column  of 
water  was  sustained  by  the  downward  pressure  of 
air.  Air,  water,  and  all  other  fluids,  exert  pressure 
in  all  directions,  while  solid  bodies  exert  downward 
pressure  only.  But  downward  pressure  means 
weight ;  hence  fluids  and  solids  (that  is,  all  bodies,) 
have  weight,  while  fluids  have,besides  weight,  pres- 
sure in  all  directions. 

familiar  facts. — From  an  open  faucet  in  a  full 
barrel  with  its  bung-hole  closed,  the  liquid  does  not 
flow,  because  the  air  presses  against  the  opening  in 
the  faucet.  To  draw  vinegar,  or  any  other  liquid 
from  a  barrel,  plunge  a  long  tube  into  the  liquid ; 
close  the  upper  end  with  the  thumb  and  withdraw 
the  tube.  The  liquid  in  the  tube  will  not  flow  out 
(why  not  ?)  as  long  as  the  thumb  closes  the  upper 
end.  Oil-cans  must  be  opened  on  the  top  in  order 
to  obtain  a  ready  flow. 

Application. — The  Barometer  (see  next  lesson). 
Pneumatic  Railway. 


48  FIRST  LESSONS   IN   PHYSICS. 


LESSON     XII. 

THE    BAROMETER. 

In  the  two  preceding  experiments  the  downward 
pressure  of  air  supported  a  column  of  water  in  a 
tumbler,  or  in  a  glass  tube.  Nothing  was  said, 
however,  concerning  the  height  of  the  liquid  col- 
umn. But  we  know  with  certainty,  that  if  a  pump 
could  be  made  with  perfect  valves  it  might  raise 
a  column  of  water,  nearly  34  feet  high.  That  is  to 
say,  the  pressure  of  the  atmosphere  might  then  sup- 
port a  column  of  water  about  34  feet  high. 

But  what  if,  by  the  same  pump,  we  had  to  lift  a 
fluid  which  has  greater  specific  gravity  than  water, 
say  liquid  tar,  or  mercury  ?  Would  the  atmosphere 
be  competent  to  support,  e.  g.,  a  column  of  mercury 
34  feet  high  ?  The  answer  is :  No.  Such  a  column 
weighs  more  than  a  column  of  water  of  the  same 
thickness ;  hence  its  height  •  must  be  less.  The 
truth  of  this  Can  be  proved  as  follows:  On  each 
of  the  scale-pans  of  a  balance  place  a  tall  vessel 
34  feet  high,  but  only  one  square  inch  across.  Both 
vessels  are  to  have  equal  weights ;  hence  the  beam 
of  the  balance  will  remain  horizontal.  Now,  fill 
one  of  them  with  pure  water  to  the  height  of  34  feet, 
and  the  other  with  mercury  until  it  balances  the 
water;  the  mercury  will  then  be  found  to  be  nearly 


THE    BAKOMETEK.  49 

30  inches  high.  We  have  now  two  columns  of 
equal  weight  and  thickness,  but  of  different  heights. 
The  specific  gravity  of  the  mercury  (Lesson  II)  is 
13.6,  and  the  height  of  the  column  of  mercury  =  34 
feet  divided  by  13.6  =  30  inches. 

But  it  was  stated  above,  that  the  atmosphere 
could  support  a  column  of  water  no  higher  than 
about  34  feet,  consequently,  the  atmosphere  must 
be  competent  to  support  a  column  of  mercury  of 
egual  weight ,  that  is,  a  column  of  mercury  about  30 
inches  high. 

In  order  to  test  this  reasoning,  take  a  glass  tube 
about  36  inches  long,  closed  at  one  end ;  fill  it  with 
mercury,  close  the  open  end  of  the  tube  with  the 
finger,  and  invert  the  tube  into  a  small  cup  or 
basin  containing  the  same  fluid.  On  withdrawing 
the  finger,  it  will  be  found  that  the  mercury  remains 
suspended  in  the  tube  to  the  height  of  about  30 
inches  (see  frontispiece,  first  figure  to  the  left).  The 
space  over  the  top  of  the  mercury  is  entirely  empty ; 
it  is  a  vacuum.  We  have  here  the  weight  of  a  col- 
umn of  mercury  completely  balanced  by  a  column 
of  air  of  the  same  weight,  but  of  indefinite  height. 
Any  change  in  the  weight  of  the  air  will  be  instantly 
followed  by  a  corresponding  change  in  the  weight 
of  the  column  of  mercury. 

From  the  preceding  it  appears,  1st,  that  a  column 
of  water  34  feet  high,  or  one  of  mercury  30  inches 
high,  nearly,  can  be  supported  by  the  pressure  of  a 
column  of  air ;  3d,  that  the  weight  and  thickness  of 


50  FIRST   LESSONS   IN   PHYSICS. 

both  columns  are  the  same,  while  the  height  of  the 
column  of  air  has  not  yet  "been  determined ;  3d,  that 
any  change  in  the  pressure  of  the  atmosphere  must 
produce  a  corresponding  change  in  the  height  of 
the  column  of  mercury. 

The  Barometer  is  an  instrument  for  ascertaining 
the  pressure  of  the  air.  Its  ordinary  form  is  that 
of  an  inverted  tube  (see  frontispiece,  centre  figure), 
with  its  open  or  lower  end  either  bent  upward  a  few 
inches,  or  immersed  in  a  small  cistern  filled  with 
mercury.  This  end  may  be  in  a  wooden  or  metallic 
case  containing  a  fine  opening  for  the  air  to  pene- 
trate to  the  mercury.  The  height  of  the  fluid  in  a 
barometer  is  the  same,  whether  the  instrument  be 
in  the  open  air  or  within  the  house.  For  air  presses 
in  all  directions  (Lesson  XI) ;  any  difference  in 
pressure,  therefore,  would  immediately  be  equal- 
ized. 

Uses  of  the  Barometer.  —  The  atmosphere  is  an 
ocean  surrounding  the  earth.  Its  pressure,  there- 
fore, must  be  greatest  at  its  greatest  depth,  viz. :  at 
the  level  of  the  sea.  It  will  be  gradually  diminished 
as  we  ascend  in  a  balloon,  because,  in  this  case,  we 
leave  air  below  us  which  does  not  press  upon  the 
mercury  in  the  balloon.  The  mercury  of  the 
barometer  actually  falls  in  proportion  to  the  eleva- 
tion to  which  it  is  taken.  Hence  the  barometer  is 
used  for  measuring  the  heights  of  mountains. 

If  the  records  of  the  barometer  at  different  places, 
but  at  identical  times,  be  compared,  probabilities 


THE    BAROMETER.  51 

may  be  obtained  in  regard  to  the  weather  which  we 
are  about  to  have.  Thus,  if  a  surplus  of  air  exists 
at  A  and  B,  the  air  in  these  localities  has  more 
weight,  and  the  barometer  will  stand  high ;  at  once 
a  quantity  of  air  will  be  conveyed  to  some  distant 
place  C,  where  the  quantity  of  air  is  deficient,  that 
is,  where  the  barometer  stands  low.  If  the  records 
show  that  at  C  the  air  was  previously  moist  and 
warm,  while  at  A  and  B  it  was  cold,  the  probabili- 
ties for  C  are  clouds,  or  rain.  Hence  the  barometer 
is  used  to  some  extent  for  ascertaining  the  weather 
that  is  about  to  follow. 

Amount  of  Pressure  of  Air. — In  the  experiment 
indicated  on  page  48,  a  weight  of  15  pounds  may, 
at  the  level  of  the  sea,  be  substituted  for  either  col- 
umn without  seriously  disturbing  the  balance.  Thus 
a  column  of  mercury  one  inch  square  and  30  inches 
high  ,  or  one  of  water  one  inch  square  and  34  feet 
high,  weighs  15  pounds.  Consequently,  the  column 
of  air  which  can  support  either  —  that  is,  one  which 
is  one  inch  square  but  reaches  to  the  top  of  the 
atmosphere,  also  weighs  15  pounds. 


52  FIRST   LESSONS  IN   PHYSIOS. 


LESSON     XIII. 

REVIEW. 

LESSON  ix. — 

1.  Springs,  ivory,  India-rubber,   and  many  other 

bodies,  have  the  property  of  recovering  their 
former  figure  when  the  force  which  distorts 
them  ceases  to  act,  provided  the  force  be  not 
too  great.  This  property  is  called  Elasticity. 

2.  All  bodies  are  more  or  less  elastic. 

3.  Hardness  is  that  property  by  which  a  body  can 

not  be  scratched,  nor  its  particles  displaced,  ex- 
cept by  the  application  of  great  force. 

4.  Softness  is  the  property  by  which  the  parts  of  a 

body  can  be  scratched  or  displaced,  if  a  slight 
•  force  is  applied. 

5.  Brittleness  is  the  property  by  which  a  body  is 

easily  fractured. 

6.  Ductility  is'  the  property  by  which  a  body  can 

be  drawn  out  into  wire. 

7.  Malleability  is  the  property  which  enables  us  to 

hammer  a  body  into  thin  sheets. 

8.  Elasticity  is  displayed  only  when  the  force  dis- 

placing the  parts  of  a  body  is  within  certain 
limits. 

9.  When  the  force  displacing  $ie  parts  of  a  body  is 

beyond  certain  limits,  the., body  may  exhibit 
brittleness,  ductility,  or  malleability. 


KEVIEW.  53 

LESSON  x. — 

10.  Air  is  an  elastic  body  ;  and  the  more  we  com- 
press it,  the  greater  is  its  expansive  force. 

LESSON  xi. — 

11.  Air,  like  water  and  other  fluids,  exerts  pressure 
in  all  directions. 

LESSON  xii. 

12.  The  downward  pressure  of  air  can  support  a 
column  of  any  other  fluid,  and  the  height  of 
this  column  depends  upon  the  specific  gravity 
of  the  fluid. 

13.  The    downward  pressure  of  the   air  is  nearly 
fifteen  pounds  to  the  square  inch  of  surface. 

14.  A  column  of  air  extending  from  the  earth  to  the 
top  of  the  atmosphere  will  support  a  column  of 
mercury  about  30  inches  high. 

15.  An  entirely  empty  space  is  called  a  vacuum. 

16.  The  barometer  is  an  instrument  for  determining 
the  pressure  of  the  air. 

17.  The  uses  of  the  barometer  are,  first,  to  measure 
the  heights  of  mountains ;  second,  to  assist  in 
ascertaining  the  weather  about  to  come. 


INERTIA. 

LESSON    XIY. 

INERTIA. 

31.  EXPERIMENT. — Place  a  piece  of  chalk  upon 
a  book,  and  move  the  book  quickly  sideways. 
The  chalk  drops  to  the  floor  without  participating 
in  the  motion  of  the  book,  because  the  book  is 

withdrawn  from  under  it. 

i 
Familiar  Facts. — A  coin  laid  upon  a  card  on 

the  mouth  of  a  bottle,  drops  into  the  bottle  if  the 
card  is  snapped  off  quickly  It  falls,  because  its 
support,  the  card,  has  been  removed  from  under 
it.  Persons  in  a  horse-car  are  thrown  backward 
if  it  starts  suddenly.  These,  and  numerous  other 
facts,  attest  that  a  body  at  rest  remains  at  rest 
until  it  is  set  in  motion  by  some  force. 

32.  EXPERIMENT.— The  motion  given  to  the 
book,  and  to  the  card,  was  so  sudden  that  there 
was  not  sufficient  time  for  it  to  be  communicated 
to  the  chalk  and  coin.   Hence  it  was  that  these  two 
bodies  did  not  participate  in  the  motion,  but  drop- 
ped simply  because  they  were  left  unsupported 
(Lesson  I).     Now  move  the  book  and    the  card 
slowly;  the  two  objects  upon  them  will  partici- 
pate in  the  motion,  and  not  fall.    This  shows  that 
to  set  a  body  in  motion,  time  is  necessary. 


INERTIA.  55 

Familiar  Facts. — A  person  running  rapidly,  or 
a  railway  train  in  motion,  cannot  stop  suddenly. 
Place  a  piece  of  chalk  on  a  book,  and  move  the 
book  until  it  strikes  against  the  wall ;  the  chalk 
will  continue  to  move  after  the  book  has  ceased 
moving.  So  do  persons  in  a  car  which  stops  sud- 
denly. A  bell  continues  to  ring  for  a  time  after  it 
has  been  struck. 

A  'boat  moves  on  a  little  after  the  action  of  the 
oars  has  ceased.  So  coffee,  when  stirred,  will  re- 
volve in  the  cup,  although  the  spoon  has  been 
removed.  In  winter,  young  persons  when  coasting 
may  be  frequently  seen  flying  down  hill  on  their 
sleds,  and  sometimes  pass  up  an  opposite  hill  a 
short  distance.  A  rabbit  cannot  run  as  fast  as  a 
hound ;  but  if  pursued  by  the  hound,  it  may,  by 
suddenly  changing  its  course  to  the  right  or  left, 
gain  considerable  advantage  over  the  hound,  which, 
not  being  prepared  for  the  change,  must  first  over- 
come his  inertia  before  he  can  turn.  From  this  we 
see  that  a  body  once  in  motion,  remains  in  motion 
until  stopped  by  some  force  or  resistance.  To  stop 
the  motion  of  a  body,  time  is  necessary. 

Inertia  is  the  tendency  which  all  bodies  have  to 
persist  in  their  state,  whether  this  state  be  of  rest 
or  of  motion. 

Application.  —  Fly-wheels.  The  switching-off 
of  railway  trains  without  a  locomotive. 


56  FIKST   LESSONS   IN   PHYSICS. 


LESSON    XV. 

THE    INCLINED   PLANE. 

33.   EXPERIMENT. — A  ball   lying  on  a  book 

upon  a  table  will  not  fall  as  long  as  the  book  lies 
in  a  horizontal  position.  But  let  the  book  be 
raised  on  one  side,  and  the  ball  rolls  down.  It 
rolls  down  the  faster  the  higher  the  book  is  tilted. 
The  ball  presses  with  its  whole  weight  upon  the 
book ;  but  when  the  book  is  raised  a  little  on  one 
side  the  ball  presses  less,  and  begins  to  fall.  The 
higher  we  raise  the  book  the  less  will  the  ball 
press  upon  the  book,  and  the  more  rapidly  will  it 
descend.  The  surface  of  the  book,  when  raised 
on  one  side,  is  an  inclined  plane. 

'Familiar  facts. — A  wagon  descending  a  steep 
hill  need  not  be  drawn  by  the  horses ;  it  is  checked 
rather,  in  order  to  prevent  it  from  rolling  down 
too  rapidly.  When  ascending  a  hill,  the  horses 
have  a  more  difficult  task  than  on  a  level  road. 
It  is  tiresome  for  us  to  ascend  steep  stairs.  In 
loading  wagons  the  skid  is  used.  It  saves  much 
labor,  if  it  is  not  placed  too  steep.  If  tlie  wagon 
is  very  high,  the  skid  must  be  quite  long.  The 
steeper  an  inclined  plane,  the  greater  the  velocity 
of  a  body  descending  on  it;  and  the  greater  the 
force  required  to  ascend  it. 


THE   INCLINED   PLANE. 


57 


34.  EXPERIMENT.— Let  an  inclined  plane  be 
formed  by  a  board.  (See  Fig.  10.)  Now  it  makes 
a  great  difference  whether  the  ball  is  rolled  down 
from  the  middle,  or  from 
the  top  of  the  inclined 
board.  Try  both.  The 
result  will  be 


FIG.  10. 


that  the 

ball,  when  rolling  down  from  the  top,  acquires  a 
greater  velocity.  A  body  increases  in  velocity  as 
the  space  increases  through  which  it  descends. 

35.  EXPERIMENT.— Before  the  lower  end  of 
a  grooved  board  place  a  ball.  Then  let  another 
ball  be  rolled  down  the  inclined  plane,  so  that  it 
strikes  the  first  ball.  Mark  the  place  to  which 
the  latter  moves,  and  put  it  in  its  former  position 
again.  Repeat  the  experiment,  having  the  upper 


end  of  the  board  raised  a  little  higher ;  that  is, 
having  the  inclined  plane  a  little  steeper  (Fig.  11). 
The  ball  rolling  down  will  then  cause  the  first  ball 
to  move  farther,  perhaps  to  a,  having  struck  it 
with  greater  force.  This  is  owing  to  the  greater 
steepness  of  the  plane.  We  have  seen  that  the 
velocity  of  a  body  increases  with  the  inclination 
of  the  plane.  The  last  experiment  shows  that 


58  FIRST   LESSONS   IN   PHYSICS. 

the  greater  the  velocity  of  a  body  the  greater  its 
striking  force. 

Familiar  Facts, — A  bullet  thrown  with  the 
hand  inflicts  less  harm  than  one  fired  from  a  gun. 
A  ~boy  running  slowly  against  a  tree  scarcely  feels 
the  shock ;  but  by  running  against  it  rapidly,  he 
might  be  seriously  injured.  We  throw  a  mar- 
ble in  the  air  and  catch  it  again  without  being 
hurt,  but  we  should  experience  pain,  if  the  mar- 
ble were  thrown  up  very  high.  Hailstones  may 
strike  with  force  sufficient  to  break  glass,  and 
to  destroy  standing  grain.  A  boy  jumps  easily 
from  a  fence,  but  would  scarcely  dare  to  jump 
from  the  top  of  a  house. 

The  descent  of  bodies  on  the  inclined  plane 
shows  that  they  are  not  supported  by  it  with 
their  whole  weight;  if  they  were,  they  would  not 
descend.  To  say  they  are  not  wholly  supported 
means :  An  inclined  plane  overcomes  a  portion 
of  the  weight  of  bodies  upon  it.  Hence  its 

Application — 1.  To  overcome  weight:  A  road 
winding  up  a  hill.  A  skid  ;  an  obliquely-placed 
plank ;  wedges  and  axes. 

2.  To  exert  strong  pressure :    Wedges  used  in 
oil-wells,  sugar-mills,  &c.,  to  press  out  the  juice. 

3.  To  overcome  cohesion.     Our  knives,   axes, 
hatchets,   scissors,   needles,   nails,   swords,  bay- 
onets, saws,  files,  chisels,  planes,  plows,  &c.,  &c. 


THE   LEVER.  59 

LESSON    XVI. 

THE   LEVER. 

36.  EXPERIMENT. — Balance  a  rod  on  the  edge 
of  a  slate  supported  "between  two  heavy  books. 
The  rod  is  in  a  state  of  equilibrium,  because  on 
each  side  of  the  point  of  support  there  is  an 
equal  amount  of  matter.  Now,  place  the  rod  in 
such  a  manner  that  on  one  side  of  the  support  it 
shall  be  twice  as  long  as  on  the  other.  The  longer 
arm  will  descend,  because  it  contains  more  mat- 
ter. Let  us  repeat  this  slowly.  Observe,  that 
in  lifting  the  end  of  the  long  arm  with  the  hand, 
it  moves  through  a  greater  space  than  is  passed 
through  by  the  end  of  the  short  arm.  The  lengths 
of  the  two  arms  of  the  lever  are  in  the  ratio  of  1 
to  2 ;  and  the  space  passed  through  by  the  end 
of  the  long  arm  is  twice  as  great  as  that  passed 
through  by  the  other.  Notice,  also,  that  the  ends 
describe  these  unequal  spaces  in  the  same  length 
of  time ;  therefore,  the  end  of  the  long  arm  of  a 
lever  has  greater  velocity  than  the  end  of  the 
short  arm. 

But  it  was  stated  before  (Less.  XV),  that,  owing 
to  their  great  velocity,  hailstones,  although  small 
bodies,  could  acquire  great  power.  So  will  any 
small  weight  or  object,  if  it  be  given  great  veloc- 
ity. Apply  this  to  the  present  case : 


60  FIRST   LESSONS   IN   PHYSICS. 

37.  EXPERIMENT.— Place  a  heavy  weight  on 

the  short  arm  of  a  lever.  The  greater  the  length 
of  the  other  arm,  the  smaller  may  be  the  weight 
upon  it  requisite  to  lift  the  large  weight  on  the 
short  arm.  The  weight  or  pressure  to  be  applied 
to  the  long  arm  for  that  purpose  is  called  the 
Power.  Thus  the  small  weight,  with  the  great 
velocity  of  the  long  arm,  counterbalances  the 
large  weight  with  the  small  velocity  of  the  short 
arm.  A  stiff  bar  made  to  turn  on  one  point  is 
a  lever. 

The  greater  the  length  of  one  arm  of  a  lever, 
the  less  power  needs  be  applied  to  that  arm  to 
lift  the  load  on  the  other  arm. 

Question. — What  power  is  needed  in  a  lever  to 
counterbalance  the  load  on  the  short  arm  ?  The 
amount  of  power  depends,  evidently,  upon  the 
length  of  the  long  arm.  If,  as  in  the  above  case, 
it  has  twice  the  length  of  the  short  arm,  the  power 
needed  to  lift  and  counterbalance  the  load  is  one- 
half  the  weight  of  the  load.  Thus,  if  a  burden 
of  100  pounds  is  to  be  lifted  by  means  of  a  lever 
whose  long  arm  has  twice  the  length  of  the  short 
arm,  a  power  of  50  pounds  is  required ;  if  four 
times,  a  power  of  one-fourth,  or  25  pounds.  To  find 
the  power  necessary  to  lift  a  load  by  means  of  a 
lever,  divide  the  product  of  the  load  into  its  dis- 
tance from  the  point  of  support  by  the  distance 
from  the  point  of  support  to  the  place  where  the 


THE   LEVER.  61 

power  is  to  be  applied.  The  quotient=the  power 
required. 

The  important  points  in  a  lever:  1.  The  point 
of  support,  or  Fulcrum.  2.  The  load  (or  weight) 
to  be  lifted.  3.  The  power  applied. 

In  the  lever  illustrated  above,  as  well  as  in  the 
applications  given  below,  the  order  of  these  three 
points  is;  Load — Fulcrum — Power.  Levers  ar- 
ranged in  this  order  are  called  Levers  of  the  First 
Class. 

Application.— The  Steelyard;  Crowbars  j  Pump- 
handles  ;  children  teetering ;  scissors  and  shears. 


If,  in  order  to  lift  a  load,  a  laborer  supports  his 
crowbar  on  a  stone  upon  the  ground,  and  enters 
the  short  arm  of  the  lever  thus  formed  under  the 
weight ,  his  lever  is  one  of  the  first  class ;  why  ? 
But  if  he  does  not  use  the  stone  ;  if  he  simply 
rests  his  crowbar  with  one  end  on  the  ground,  so 
that  the  load  comes  to  lie  between  him  and  the 
fulcrum  (the  ground),  then  the  order  of  his  lever 
is :  Fulcrum — Load — Power ;  and  this  constitutes 
a  Lever  of  the  Second  Class. 

Application. —  The  nut-cracker;  where  its  limbs 
are  riveted  together,  is  the  Fulcruni ;  the  nut  re- 
presents the  load  (in  this  case  the  load,  or  resist- 
ance, is  to  be  crushed,  not  lifted);  the  power  is 
where  the  hands  are  applied.  We  have  here  two 
levers  combined.  The  chopping -Tinife  is  a  lever 


62  FIRST   LESSONS   IN   PHYSICS. 

of  the  same  class.  Where  the  knife  is  fastened 
is  the  Fulcrum.  The  object  to  be  cut  is  the  Load ; 
the  Power  is  at  the  handle  (in  this  case,  too,  the 
resistance  is  not  a  load  to  be  lifted,  but  cohesion 
to  be  overcome). — Lemon-squeezers  ;  Cork-squeez- 
ers ;  the  Wheel-barrow  ;l  the  oar  of  a  boat.2 

The  great  progress  of  our  age  does  not  lie  so 
much  in  the  introduction  of  new  forces  of  nature, 
of  which  there  are  but  a  few,  but  in  the  ingenious 
application  of  those  few  forces,  and  in  their  skill- 
ful combination  into  machines.  One  of  the  offices 
of  machines  is  to  communicate  the  effect  of  a  force 
to  bodies  which  otherwise  could  not  be  acted  upon 
by  that  force.  Thus,  without  the  locomotive,  the 
expansive  force  of  steam  could  not  communicate 
its  effect  upon  a  train  of  cars. 

The  Lever  is  the  simplest  of  all  machines  ;  and 
probably,  also,  the  most  ancient.  By  means  of  a 
very  long  arm,  it  becomes  a  most  powerful  instru- 
ment. It  is  told  of  Archimedes,  a  Syracusan 
philosopher  (about  250  years  before  Christ),  that 
he  offered  to  move  the  earth  itself,  if  the  king 
would  give  him  a  place  to  stand  on. 

Read  on  Levers,  and  The  Art  of  Walking,  in  "Things  Not  Generally 
Known." 

1.  The  Fulcrum  is  where  the  wheel  rests  on  the  ground. 

2.  The  Fulcrum  is  where  the  oar  rests  in  the  water ;  the  Load  is  the 
boat. 


THE   PENDULUM.  63 

LESSON     XVII. 

THE   PENDULUM. 

38.  EXPEEIMENT. — (a.)  The  string  "by  which  a 
stone  is  suspended  has  a  vertical  direction  (Less. 
I).  If  the  stone  is  drawn  a  little  to  one  side,  the 
direction  of  the  string  will  be  slanting.  On  letting 
the  stone  go  now,  it  will  begin  to  move.  Since  all 
bodies  are  drawn  to  the  earth  (Less.  I),  it  will 
approach  the  earth  as  near  as  possible.  When 
nearest  the  earth,  it  has  again  the  vertical  direc- 
tion. But  the  weight  does  not  stop  there;  its 
inertia  (Less.  XIV)  carries  it  onward  ;  being  held 
by  the  string  it  does  not  fall  to  the  ground ;  it 
ascends,  until  gravity  finally  stops  it.  Gravity 
not  only  stops  it,  but  also  pulls  it  down  again. 
Noticing  its  downward  course  more  closely,  we 
see  that  it  descends  with  increasing  velocity. 
Inertia  causes  it  again  to  pass  by  the  lowest  point 
of  its  path ;  it  ascends  on  the  other  side,  stops  an 
instant  of  time,  and  is  then  forced  back  again  by 
gravity.  Thus  it  swings  back  and  forth  for  a 
certain  time.  Each  swinging  in  one  direction  is 
called  a  vibration.  The  vibrations  grow  shorter, 
and  observation  shows  us  that,  finally,  they  cease 
al  together. 

If,  while  the  pendulum  is  vibrating,  we  beat 
time,  it  will  be  found  that  the  same  length  of  time 
is  necessary  for  the  shorter  vibrations  toward  the 


64  FIRST   LESSONS   IN   PHYSICS. 

close  of  the  experiment  as  for'  the  earlier,  longer 
ones.  Thus  if  the  pendulum  at  first  made  sixty 
vibrations  a  minute,  it  will  continue  to  make  the 
same  number  during  the  same  time,  although  it 
afterward  passes  through  shorter  arcs. 

The  vibrations  of  the  same  pendulum  will  talce 
place  in  the  same  lengtli  of  time,  unless  these  vibra- 
tions pass  through  large  spaces  or  arcs. 

39.  EXPERIMENT.  —  (&.)  Cut  off  three-fourths  of 

the  string.  The  pendulum  is  now  shorter  than  it 
was  before ;  it  has  only  one-fourth  the  former 
length.  After  it  is  set  vibrating,  count  the  num- 
ber of  its  vibrations  in  ten  seconds  ;  the  number 
will  be  greater  than  that  of  the  former  pendulum.* 
The  reason  of  this  is  easily  seen,  if  we  suspend  the 
shorter  and  longer  pendulum,  both,  from  the  same 
horizontal  height.  The  former  descends  on  a  shorter 
incline  than  the  latter,  and,  therefore,  takes  less 
time  to  descend.  This  shows  that  a  short  pendu- 
lum vibrates  more  rapidly  than  a  long  one} 

I  A  pendulum  which  is  four  times  as  long  as  another  will  need  twice 
as  much  time  to  perform  one  vibration ;  that  is,  it  will  vibrate  half  as  fast 
as  the  other.  Let  one  pendulum  be  nine  times  as  long  as  the  other,  it  will 
need  three  times  as  much  time  (it  will  vibrate  one-third  as  fast)  ;  or,  it  will 
vibrate  once  while  the  other  vibrates  three  times.  Hence  the  times  of 
vibrations  of  pendulums  are  to  each  other  as  the  square  roots  of  their 
lengths.  Thus,  if  one  pendulum  has  a  length  of  4,  and  another  the  length 
of  36,  the  former  will  vibrate  faster  than  the  latter ;  the  square  roots  being 
2  and  6,  the  latter  will  require  three  times  as  much  time  as  the  other  to 
perform  one  vibration ;  that  is,  if  it  vibrates  once  every  three  seconds,  the 
former  will  vibrate  once  every  one  second;  or,  the  longer  pendulum  will 
vibrate  once  in  the  same  length  of  time  that  the  shorter  one  vibrates  three 
times. 


THE   PENDULUM. 


65 


The  principal  application  of  the  pendulum 

is  its  use  for  the 
regulation  of 
clocks.  A  clock  is 
required  to  keep 
good  time.  From 
the  preceding,  it  is 
evident  that  a  pen- 
dulum, by  its  vi- 
brations, whether 
it  moves  fast  or 


slowly,  gives  us 
equal  portions  of 
time.  But  a  pen- 
dulum will  cease 
vibrating  after  a 
short  time ;  what, 
then,  must  be  done 
to  meet  this  diffi- 
culty? Everybody 
FIG-12-  knows  that  the 

pendulum  stops  when  the  clock  has  "  run  down ; " 
that  is,  when  the  weight  has  reached  its  lowest 
point.  It  is  plain  that  the  downward  tendency 
of  the  weight  is  quite  sufficient  to  meet  that 
difficulty  ;  for  while  the  pendulum  alone  would 
very  soon  cease  vibrating,  the  descent  of  the 
weight  lasts  at  least  twenty-four  hours.  (What  is 
meant  by  winding  up  a  clock  ?)  But  the  weight, 
after  it  begins  to  descend,  increases  in  speed  (Les- 
son XV),  and  as  the  cord  from  which  it  is  sus- 
5 


66 


FIKST   LESSONS   IN   PHYSICS. 


pended,  passes  round  an  axle  which  causes  the 
hands  to  move,  the  accelerated  velocity  would 
cause  the  hands  to  move  faster  and  faster.  To 
obviate  this,  the  axle  is  connected  with  a  wheel 
of  saw-shaped  teeth  (Fig.  12),  which  revolves  with 
it,  and  above  which  swings  a  curved  hook,  A.  A., 
called  an  escapement,  whose  two  teeth  work  al- 
ternately in  the  saw-shaped  teeth  of  the  wheel. 
At  every  vibration  of  the  pendulum,  one  of  these 
two  teeth  stops  the  revolution  of  the  wheel,  and 
thus  interrupts  the  descent  of  the  weight.  Now, 
since  the  pendu- 
lum vibrates  in 
equal  portions  of 
time,  the  weight 
descends  through 
equal  spaces  in 
equal  times.  And 
since  the  weight 
descends  through 
equal  s  p  a  c  e  s  in 
equal  times,  it 
turns  the  axle,  the 
work,  and  the 
hands  with  uni- 
form velocity. 
Hence  clocks  are 
moved  by  the  de- 
scent of  the  weights,  and  regulated  by  the  vibra- 
tions of  the  pendulum.  (See  Fig.  13.) 


COMMUNICATING  VESSELS.  67 


LESSON    XVIII. 

COMMUNICATING    VESSELS — HYDRAULIC   PRESS. 

40.  EXPERIMENT.  —  Fit  a  piece  of  thin  board 

into  a  tumbler,  as  a  vertical  partition  divid- 
ing the  inside  of  the  tumbler  into  two  spaces. 
The  board  should  not  touch  the  bottom  of  the 
glass,  but  be  a  little  above  it.  Now  pour  water 
into  the  tumbler,  and  there  will  be  two  horizontal 
surfaces  of  water,  each  having  the  same  height. 
Remove  the  board,  and  in  place  of  it  immerse  a 
wide  glass  tube.  The  two  surfaces  of  water  will 
again  be  of  the  same  height,  nearly. 

Familiar  Facts. — The  same  may  be  seen  in 
two  glass  tubes  of  unequal 
width  (Pig.  14)  which  are  ce- 
mented into  a  base  made  of 
tin,  and  connected  with  each 
other  by  means  of  a  tin  tube 
(a).  Also  in  a  teapot.  The  tea 
PIG.  u.  rises  as  high  in  the  spout  as  in 

the  body  of  the  pot,  and  if  the  body  were  higher 

than^the  spout  the  tea  would  flow  from  the  spout. 

Hence,  in  pouring  out  tea,  we  lift  the  pot  and 

lower  the  spout. 

41.  EXPERIMENT.— Take  a  tube  made  of  glass 
or  tin  and  bend  it  so  that  one  limb  be  very  short, 
perhaps,  only  one-twentieth  as  long  as  the  other, 


68  FIRST   LESSONS   IN   PHYSIOS. 

and  let  the  opening  of  the  short  limb  be  drawn 
out  fine  (Fig.  15).  Then  pour  water  into  the 
long  tube,  holding  the  short  one  closed  with 
the  finger.  On  removing  the  finger,  water 
will  jet  forth.  Thus  we  have  a  fountain  on 
a  small  scale.  If  the  short  tube  were  tall 
enough,  the  water  would  rise  until  it  stood 
at  a  level  with  the  water  in  the  other  tube. 
The  tube  being  short,  however,  the 
water  rises  in  a  jet,  but  not  to  that  level, 
because  there  is  friction,  and  because  the 
returning  drops  depress  the  rising  jet. 

Familiar  Facts. — Cisterns,  offices,  dwel- 
ling-houses and  factories  are  supplied  with 
water  from  large  elevated  reservoirs. 

Vessels  connected  with  each  other,  so  that  a 
liquid  'can  pass  freely  from  one  into  the  other, 
are  called  Communicating  Vessels. 

Why  may  water  pipes  under  ground  be  said  to 
be  communicating  tubes  ? 

42.  EXPERIMENT.— Take  a  cylindrical  tin  ves- 
sel (about  five  inches  high),  with  a 
neck,  B,  perfectly  cylindrical  (Fig.  16), 
into  which  a  cork  can  be  fitted  tightly, 
and  with  small  holes  in  the  sides  of 
the  vessel  as  well  as  in  the  upper 
(tapering)  part.  These  openings  are 
carefully  closed  with  beeswax,  the 
vessel  filled  with  water  to  the  very 


HYDEAULIO   PEESS.  69 

edge  of  B,  and  the  cork  set  on  the  neck.  If  the 
cork  is  then  driven  in  by  a  sudden  "blow  with  the 
hand,  the  water  jets  forth  from  all  the  openings 
simultaneously. 

One  end  of  a  glass  tube  is  cemented  into  a  small, 
hollow  tin  ball  provided  with  about 
a  dozen  fine  openings.  (See  wood, 
cut.)  The  other  end  is  freely  in- 
serted in  a  rubber  ball,  previously  filled  with  water. 
When  the  ball  is  pressed  with  the  hand,  the  same 
phenomenon  as  above  is  witnessed.  (To  fill  the 
ball,  squeeze  it  together  under  water,  and  then  let 
it  go.) 

The  force  of  a  pressure  brought  to  bear  upon  a 
small  portion  of  a  liquid,  is  transmitted  equally 
to  all  parts  of  the  liquid. 

Suppose,  now,  that  the  bottom  of  the  tin  vessel  had 
merely  been  telescoped  in  the  vessel.  The  pressure 
given  to  the  water  in  B  would  evidently  have 
forced  the  bottom  out ;  and  the  bottom  would  then 
have  exerted  a  pressure  upon  any  resisting  object. 

Application.  —  Advantage  has  been  taken  of 
this  in  a  machine  called  the  "Hydraulic  Press," 
invented  in  1796  (Fig.  17).  By  means  of  a  lever 
(of  the  second  kind)  a  pressure  is  exerted  upon 
the  water  in  the  narrow  tube,  A.  This  pressure  is 
communicated  to  the  water  in  the  wide  tube,  (7, 


70 


FIRST  LESSONS   IN   PHYSICS. 


forcing  the  movable  cylinder,  J5,  to  ascend.  Bales 
of  cotton,  or  any  other  object  to  be  compressed, 
lying  on  the  plate,  and  prevented  from  rising 
by  the  fixed  plate,  P,  are  thus  compressed  with 


PIG.  17. 

enormous  force.  For  if  the  surface  of  the  water 
in  the  cylinder  be  100  times  that  of  the  water  in 
the  narrow  tube,  and  if  the  pressure  applied  to 
the  liquid  in  the  tube  amount  only  to  50  pounds, 
the  pressure  exerted  upon  the  bale  of  cotton  will 
amount  to  5,000  pounds  But  since  the  power 
applied  by  the  hand  may  be  increased  tenfold 
with  the  advantage  gained  by  a  longer  lever,  the 
amount  of  pressure  may  easily  be  raised  to 
50,000  pounds  It  can  be  farther  increased  by 
steam-pressure  so  that  the  force  of  pressure  may 
amount  to  over  a  million  pounds. 


BREATHING. — THE  BELLOWS.  71 


LESSON"    XIX. 

BREATHING. — THE   BELLOWS. 

43.  EXPERIMENT. — If  a  glass  tube  be  placed 
with  one  end  in  water,  we  can  cause  the  water  to 
rise  in  the  tube  by  sucking  it  up  with  the  mouth. 
This  is  the  reason  for  it :  We  draw  the  air  which 
is  in  the  tube,  into  the  mouth ;  a  vacuum  (Lesson 
XIII)  is  thus  created,  and  the  pressure  of  the  ex- 
ternal air  upon  the  water  forces  water  into  the 
tube. 

Familiar  Facts.—  Instead  of  water  we  may 
draw  up  air  alone  ;  this  is  done  in  breathing.  We 
enlarge  our  lungs  and  the  cavity  in  our  chest 
(Lesson  II,  p.  16) ;  by  this,  the  air  in  the  chest  is 
rarefied,  and  the  external  air,  by  the  pressure  of 
the  layers  of  air  above  it,  forced  to  rush  into  the 
chest.  This  process  is  called  Inspiration.  During 
the  process. of  Expiration  we  contract  the  chest, 
and  the  air  must  rush  out.  If  we  immerse  a  pail 
in  a  pond,  and  fill  it  with  water,  the  moment  the 
pail  is  drawn  out  again,  the  water  rushes  in  and 
occupies  the  space  where  the  pail  was  before.  In 
the  same  way 

Air  rushes  into  a  vacuum,  or  into  any  space 
containing  rarefied  air. 


72  FIKST   LESSONS   IN  PHYSICS. 

2-.  very  useful  application  of  the  pressure  of 
air  is  the  Bellows,  an  instrument  for  blowing  fire. 
It  consists  of  a  space  enclosed  by  two  boards 
opposite  each  other,  which  are  united  around  the 
edges  by  a  wide  strip  of  leather.  In  front,  this 
spaco  opens  in  a  narrow  tube.  In  one  of  the 
boards  is  a  hole  closed  by  a  valve.  A  valve  is  a 
sort  of  lid  or  cover,  which  admits  a  fluid  into  a 
space,  but  prevents  its  return.  When  the  bel- 
lows is  drawn  out,  the  air  inside  is  rarefied.  The 
external  air  now  seeks  to  rush  in,  but  it  finds  no 
other  way  than  through  the  valve ;  this  it  opens 
and  instantly  fills  the  extended  bellows.  When 
the  bellows  is  drawn  in,  the  air  inside  is  com- 
pressed, and  its  expansive,  or  elastic,  force  (Les- 
son X)  being  greatly  increased,  it  presses  against 
the  inner  sides  of  the  bellows,  and,  in  doing  so, 
closes  the  valve.  There  being  no  other  egress, 
the  air  passes  through  the  tube  in  front  into  the 
fire. 

On  the  same  principle  yon  may  explain  Drink- 
ing and  Smoking. 


REVIEW.  73 


REVIEW. 

LESSON  xv. — 

1.  The  steeper  an  inclined  plane,  the  greater  the 

velocity  of  bodies  descending  on  it ;  and  the 
greater  the  force  required  to  ascend  it. 

2.  A  body  increases  in  velocity  as  the  space  in- 

creases through  which  it  descends. 

3.  The  greater  the  velocity  of  a  body,  the  greater 

its  striking  force. 

LESSON  xvi. — 

4.  A  lever  is  an  inflexible  bar  made  to  turn  about 

a  fixed  point. 

5  When  moving,  the  end  of  the  long  arm  (where 
the  power  is  applied)  has  greater  velocity 
than  the  end  of  the  short  arm  where  the  load 
is  attached. 

6.  The  greater  the  length  of  the  long  arm  of  the 

lever,  the  greater  becomes  its  velocity;  and, 
consequently,  the  less  power  needs  be  ap- 
plied to  lift  the  load. 

7.  To  find  the  power  required  to  lift  a  load  by 

means  of  a  lever,  divide  the  product  of  the 
load  into  its  distance  from  the  point  of  sup- 
port by  the  distance  between  the  point  of 
support  and  the  place  where  the  power  is  to 
be  applied.  The  quotient  is  the  Power. 


74 


FIRST   LESSONS   IN   PHYSICS. 


LESSON    XX. 

COMMON    PUMP. 

44.  EXPERIMENT. — Instead  of  immersing  the 
end  of  a  glass  tube,  as  we  did  in  the  preceding 
lesson,  let  us  dip  a  syringe  into  water.  On  draw* 
ing  up  the  piston,  by  means  of  the  piston-rod,  the 
liquid  is  seen  to  rise  in  the 
syringe.  This  is  explained 
by  the  law  given  in  the  pre- 
ceding lesson,  for  the  piston 
being  air-tight,  it  leaves,  as 
it  rises,  a  vacuum  below  it, 
which  is  eagerly  filled  by 
the  water.  But  what  causes 
the  water  to  rise  ?  The  answer 
is :  The  pressure  of  air  on  the 
surrounding  water. 

Application. — Our  pumps. 
They  act  on  the  same  princi- 
ple. When  we  look  at  a 
pump  (Fig.  18),  the  first  thing 
that  strikes  our  eye  is  the 
cylinder,  or  barrel,  C,  the 
spout,  8,  and  the  lever,  or 
handle,  H.  The  lower  part 
of  the  barrel,  P,  is  called  the 


PIG.  18. 


COMMON   PUMP.  75 

suction-pipe  ;  it  is  submersed  in  the  water.  Inside 
of  the  barrel  works  a  piston  0,  which  fits  air-tight, 
and  can  be  moved  up  and  down  by  means  of  the 
piston  rod  to  which  it  is  attached.  It  is  pierced 
with  a  hole,  and  the  hole  is  covered  by  a  valve,  t), 
which  opens  upward.  The  piston-rod  is  connected 
at  the  top  with  the  lever  H. 

When  the  handle  of  the  pump  is  raised,  and  has 
arrived  at  its  highest  point,  the  piston  is  at  its 
lowest,  directly  over  the  valve  A. 

Let  us  see  now  what  happens  when  the  handle  is 
being  forced  down,  as  in  Fig.  18.  First,  observe 
that  the  piston  rises  ;  next,  that  the  valve  A  opens. 
But  what  causes  this  valve  to  open  ?  The  answer 
is :  When  the  piston  rises,  the  air  previously  con- 
fined between  valve  A  and  the  level  of  the  water 
in  the  suction-pipe  now  occupies  a  larger  space ; 
hence  it  is  rarefied,  and  has  less  pressure  than  the 
air  over  the  outside  water  F  F.  The  result  of  these 
unequal  pressures  is  that  the  level  of  F  F  is  lowered 
by  the  pressure  of  the  atmosphere  over  it,  and  that 
the  water  in  the  suction-pipe,  P,  must  rise  until  the 
air  inside  of  the  tube  has  reached  the  same  pressure, 
nearly ;  that  is,  the  same  degree  of  density  as  the 
outside  air.  At  this  instant,  the  valve  A  will  close-  • 
that  is,  it  will  fall  of  its  own  weight.  It  is  evident 
that  the  valve,  V,  remains  closed  whenever  the  piston 
rises. 

When  the  piston  is  lowered,  the  valve  v  is  forced 
open.  Why  ?  Because  the  air  below  the  piston  is 


76  FIRST   LESSONS   IN   PHYSIOS. 

compressed,  and,  remembering  p.  44,  we  know  that 
it  is  the  expansion  of  the  compressed  air  which 
opens  the  valve  v.  When  the  piston  is  at  its  lowest 
this  valve  falls  by  its  own  weight.  On  raising  the 
piston-rod  the  second  time,  more  air  is  withdrawn 
from  the  suction-pipe;  water  commences  rushing 
up,  and  enters  through  valve  A.  On  lowering  the 
piston  again,  it  descends  into  the  water,  and  from 
this  moment  all  the  air  below  the  piston  is  expelled. 
Some  water  is  now  above  the  piston,  and  the  lower 
valve  again  falls  of  its  own  weight.  Henceforth, 
whenever  the  piston  descends,  a  large  quantity  of 
water  passes  through  the  piston  valve  v;  whenever 
it  rises,  that  quantity  of  water  remains  on  the  top 
of  the  piston-valve.  Afterward,  at  every  rise  of  the 
piston,  the  water  above  it  flows  out  through  the 
spout. 

The  great  principle  of  the  pump  is  the  fact,  that 
the  pressure  of  the  air  upon  a  body  of  water,  causes 
the  water  to  rush  up  into  a  vacuum  that  lias  been 
formed  in  a  tube  communicating  with  that  body  of 
water. 

The  column  of  water  in  the  suction-pipe,  between 
the  level  of  F  F  and  valve  7,  is  supported  by  the 
pressure  of  the  air  on  the  water  in  the  cistern. 

Bead  "  Theory  of  Pump  "  p.  267,  in  "  Things  not  Generally  Known." 

7" 


FORCING   PUMP. — FIRE   ENGINE. 


77 


LESSON   XXI. 

FORCING   PUMP. — FIRE   ENGINE. 

A  common  pump  cannot  draw  water  to  a  ver- 
tical height  of  34  feet,  because  its  vacuum  cannot 
be  made  perfect. 

In  order  to  elevate  it  to  a  greater  height,  the  Forc- 


ing Pump  is  used  (Fig.  19).     It  is  constructed  on 


78  FIEST   LESSONS   IN   PHYSICS. 

the  same  principle  as  the  Common  Pump ;  it  dif- 
fers from  the  latter  in  the  following  three  points  : 

1.  The  piston  of  the  Forcing  Pump  is  not  pierced. 

2.  In  place  of  the  spout  there  is  a  tube  at  the 
lower  part  of  the  barrel,  which  leads  to  the  place 
wnere  the  water  is  to  be  carried.    3.  That  tube 
contains  a  valve,  a2,  which  opens  outwardly. 

When  the  piston  P  is  raised  this  valve  is  closed; 
thus  the  air  below  the  piston  becomes  rarefied,  and 
water  is  drawn  through  the  lower  valve  u1,  the 
same  as  in  the  common  pump.  When  the  piston 
descends,  the  lower  valve  is  closed  on  account  of 
its  own  weight ;  the  water  above  the  valve  v1  is 
then  forced  through  valve  vz  into  the  tube,  from 
which  it  can  not  flow  back.  (Why  not  ?) 

The  Fire-Engine 

Consists  of  a  Heron 's  Fountain  (Lesson  X)  and 
of  two  Forcing  Pumps  to  pump  water  into  it. 
Both  pumps  stand  in  a  large  box  filled  with 
water.  Two  iron  levers  (called  brakes)  L  and  L\ 
work  the  iron  piston-rods  P  and  P1.  A  wide 
cylinder,  N^  stands  between  the  two  pumps.  It 
contains  air,  and  a  metallic  tube  which  nearly 
reaches  to  the  bottom  and  is  open  at  the  top. 
This  cylinder  acts  like  a  Heron's  Fountain,  but  in 
the  Fire-Engine,  and  in  other  pumps,  it  is  called 
an  Air-  Chamber.  The  tubes  of  the  Forcing  Pumps 
enter  the  air-chamber ;  each  has  a  valve  opening 
outwardly  into  the  air-chamber. 


FORCING  PUMP. — FIRE-ENGINE. 


79 


FIG. 


When  A,  one  of  the  pistons,  rises,  the  valve  of 
tube  B  is  closed  by  the  pressure,  which  the  air 
over  the  water  in  the  air-chamber  exerts.  Water, 
at  the  same  time,  enters  from  the  box  through 
the  lower  valve  C  into  the  barrel  of  the  pump. 
Why?  When  the  piston  E  descends,  the  lower 
valve  D  closes  of  its  own  weight,  and  water  is 
forced  into  the  air-chamber  through  the  valve 
of  the  tube  F.  After  continued  pumping,  the 
water  in  the  air-chamber  has  risen  so  high  that 
it  has  concentrated  and  compressed  the  air  into 
a  much  smaller  space.  But  from  Lesson  X  we 
see  that  the  more  we  compress  air,  the  greater  its 
expansive  force.  Hence  it  is  evident  that  the  jet 


80  FIRST   LESSONS  IN   PHYSICS. 

of  water  sent  forth  from  the  metallic  tube  is  sent 
forth  by  the  expansive  force  of  the  compressed 
air  in  the  air-chamber.    There  being  two  pumps 
and  an  air-chamber  the  jet  is  continuous. 

Give  the  difference  between  the  Common  Pump 
and  the  Forcing  Pump. 

Also,  between  a  Hero's  Fountain  and  the  Air- 
Chamber  of  a  Fire-Engine. 


The  Common  Pump  and  Barometer  Compared. 

Four  points  in  common : 

1.  Both  have  a  tube  or  cylinder. 

2.  Both  have  a  vacuum. 

3.  In  both,  the  liquids  rise  in  consequence  of 
the  pressure  of  air. 

4.  In  both,  the  liquids  can  not  rise  higher  than 
the  capacity  of  that  pressure  permits. 

Seven  points  of  difference : 

1.  The  Barometer  has  a  glass  tube ;  pumps 
usually  have  iron  tubes. 

2.  The  barometer-tube  is  closed  above  the  vacu- 
um ;  while  in  pumps  there  is  a  valve  above  the 
vacuum. 

3.  The  vacuum  in  the  pump  can  never  be  made 
as  perfect  as  that  in  the  barometer. 


COMMON  PUMP  AND  BAROMETER.       81 

4.  In  the  pump,  the  vacuum  must  first  be  pro- 
duced ;  in  the  barometer,  the  vacuum,  once  estab- 
lished, remains. 

5.  Tho  liquid  in  the  barometer  is  always  mer- 
cury; in  the  pump  it  may  be  water,  oil,  vinegar, 
&c.,  &c. 

6.  In  the  barometer  neither  spout  nor  lever  is 
required. 

7.  No  graduated  scale  is  attached  to  the  pump. 

8.  The  liquid  column  in  the  barometer  usually 
stands  no  higher  than  30  inches.    The  liquid  col- 
umn in  the  pump  stands  higher  than  that  of  the 
barometer  (Comp.  Lesson  XII).  Thus  when  the  mer 
cury  in  a  barometer  reads  29'5  inches,  this  num- 
ber is  an  index  of  the  pressure  of  the  air  at  the 
time ;  and  in  a  common  pump  with  perfect  valves, 
the  water  could  then  be  drawn  up  29*5  x  13*6  = 
401-2   inch.  =  33-4  feet. 

The  distance  between  the  level  of  the  water  in 
the  cistern  and  the  lower  valve  must  be  propor- 
tionate to  the  capacity  of  the  pump.  Suppose, 
for  instance,  it  were  forty  feet  in  the  imagi 
nary  pump  previously  mentioned,  then  the  water 
would  not  come  up  to  the  lower  valve,  because  the 
atmospheric  pressure  cannot  lift  a  column  of 
water  higher  than  its  natural  limit,  explained  in 
Lesson  XII. 
6 


82  FIRST   LESSONS   IN   PHYSICS. 

LESSON    XXII. 

REVIEW. 

LESSON  xvir. — 

1.  The  vibrations  of  the  same  pendulum  will  take 

place  in  the  same  length  of  time,  unless  these 
vibrations  pass  through  large  spaces  or  arcs. 

2.  A  short  pendulum   vibrates   more  quickly — 

makes  a  greater  number  of  vibrations  in  the 
same  length  of  time — than  a  longer  one. 
3  In  Clocks  the  motory  force  is  the  force  of 
Gravity  ;  in  Watches  (and  in  clocks  without 
weights),  the  motory  force  is  the  force  of  Elas- 
ticity. 

4.  In  Clocks  the  motion  is  regulated  by  the  Pen- 

dulum ;  in  Watches  by  the  Balance- wheel. 
LESSON  xvin. — 

5.  Vessels  connected  with  each  other,  so  that  a 

liquid  can  pass  freely  from  one  into  the  other, 
are  called  Communicating  Vessels. 

6.  The  force  of  pressure  upon  a  small  portion  of 

any  liquid  is  transmitted  equally  and  undi- 
minished  to  all  the  parts  of  the  liquid  in  all 
directions. 

LESSON  xix — 

7.  Air  rushes  into  a  vacuum,  or  into  any  spa~e 

containing  rarefied  air. 


REVIEW.  83 

LESSON  xx. — 

8.  If  there  is  a  Fluid  between    a    vacuum  and 

the  air,  the  pressure  of  air  will  force  the 
Fluid  into  the  Vacuum.  Thus  water  or  mer- 
cury rushes  into  a  vacuum  formed  over  a  part 
of  its  surface,  because  the  pressure  of  air 
upon  the  remaining  portion  of  its  surface 
forces  both  to  do  so.  (Pumps  and  Barometer.) 

9.  A  stone  on  a  support,  a  weight  suspended  by 

a  cord,  are  at  rest.  They  may  remain  at  rest 
during  thousands  of  years.  The  force  of 
gravity  in  them  is  also  at  rest.  But  as  soon 
as  the  support  is  withdrawn,  or  the  cord 
lengthened  but  the  hundreth  part  of  an  inch, 
they  begin  to  move.  Then  the  force  of  gravity 
in  them  may  be  said  to  do  work.  This  work 
is  called  Motion. 

10.  An  elastic  spring  may  be  compressed,  and 
may  remain  so  for  thousands  of  years.   Dur- 
ing this  time  the  force  of  elasticity  in  it  does 
no  work.    But  withdraw  the  pressure,  and 
the  spring  commences  moving.    Its  motion  is 
t?ie  work  done  by  the  Force  of  Elasticity. 

11.  Motion  is  a  manifestation  of  the  work  done 
by  a  Force,  and  is  always  accompanied  by  a 
cliange  of  place. 

12.  a.  A  body  on  an  incline  (Fig.  10)  will  not  fall ; 

b.  A  pendulum  (Fig.  13)  will  not  vibrate ; 

c.  The  long  arm  of  a  lever  will  not  move ; 


84  FIRST  LESSONS   IN  PHYSIOS. 

d.  The  water  in  the  wide  tube  of  a  communi- 
cating vessel  (if  by  means  of  a  stop -cock 
shut- off  from  the  narrow  tube)  will  not  flow 
into  the  narrow  tube  (Fig.  14); 

e.  The  air  outside  the  bellows  will  not  enter ; 
/.  The  air  over  the  cistern  of  a  pump  will  not 

force  the  water  up  (Figs.  18  and  19) , 
— So  long  as  the  force  of  gravity  (in  e  and  / 
the  force  of  elasticity)  does  no  work.  But 
from  the  moment  that  the  rope  at  the  top  of 
the  incline,  to  which  the  body  is  fastened,  is 
cut;  from  the  moment  that  the  pendulum- 
weight  be  drawn  to  one  side ;  that  the  long 
arm  of  the  lever  be  provided  with  additional 
weight;  that  the  stop- cock  in  the  communi- 
cating-tube be  opened  ;  that  the  bellows  be  ex- 
tended ;  that  the  piston  of  the  pump  be  moved ; 
—  from  that  moment  Work  is  done  and  Mo- 
tion produced. 

IB.  The  effect  of  the  Force  of  Gravity  is  Pull.  It 
pulls  all  bodies  to  the  earth.  The  effect  of 
the  Force  of  Elasticity  is  Push  (pressure). 
These  effects  disappear  when  work  is  being 
done  by  the  forces ;  the  forces  are  then  con- 
verted into  Motion. 

14,  The  motion  of  masses  is  produced  by  the  work 
which  their  forces  perform.  The  motions  of 
the  human  body  are  work  which  its  forces 
perform.  When  its  forces  cease  to  labor, 
death  takes  place.  In  Nature  all  is  Motion, 
Life  and  Labor. 


SOUND.  85 

LESSON   XXIII. 

SOUND. 

Familiar  Facts.  —  The  passage  of  an  electric 
spark  through  the  air  is  followed  by  a  crackling 
noise,  as  the  passage  of  lightning  through  air  is 
followed  by  thunder.  The  blow  of  a  whip  in  the 
air  is  also  accompanied  by  a  crackling  noise  ;  and 
a  pencil,  when  it  falls  from  the  table,  produces  an 
audible  sound.  So  does  a  stone  thrown  into  the 
water,  a  book  dropped  upon  the  floor,  or  the  hand 
rapping  at  the  door.  Now,  if  the  whip  had  not 
moved  through  the  air,  nor  the  pencil  upon  the 
table,  nor  the  stone  into  the  water,  nor  the  book  on 
the  floor,  these  sounds  would  evidently  not  have 
been  produced. 

All  sound  is  transmitted  to  the  ear  through  tho 
air ;  no  sound  is  heard  in  a  vacuum. 

45.  EXPERIMENT.  —  Insert  the  hlade  of  a  knife 

between  the  horizontal  joints  on  the  side  of  a  desk 
or  table ;  take  the  free  end  of  the  handle,  press  it 
downward  as  far  as  convenient,  and  then  let  go  :  a 
noise  will  be  heard,  and  the  knife  will  be  seen  to 
move  up  and  down  very  fast  until  it  comes  to  rest. 
This  is  a  swinging,  or  a  vibratory  motion,  similar  to 
that  of  the  pendulum  of  a  clock. 

46.  EXPERIMENT.  —  Let  a  few  drops  c  f  water 
fall  into  a  tumbler  filled  with  water.     At  first  the 


86  FIRST   LESSONS   IN   PHYSICS. 

water  is  depressed,  but  quickly  rises  again.  This 
vibratory  motion  is  communicated  to  the  remaining 
water.  The  water  shows  it  in  the  circular  elevations 
(rings)  round  the  point  of  contact.  Thus  the  mo- 
tion of  the  knife,  as  well  as  that  of  the  water,  is  a 
vibratory  motion. 

Familiar  Facts.  —  A  vibratory  motion  may  be 
heard  and  felt,  when  a  door  is  slammed  or  a  gun 
fired  off.  A  bell  is  first  set  to  vibrate,  then  it  com- 
municates its  own  vibrations  to  the  air  around  it 


FIG    21. 

and  the  air  in  turn  transmits  its  vibrations  to  the 
ear.  On  water,  the  vibrations  are  rings  ;  in  air,  hol- 
low spheres  of  compressed  air,  alternating  with 
iollow  spheres  of  rarefied  air.  No  sound  is  heard 
if  the  vibrations  are  too  faint,  or  if  the  organ  of 
hearing  is  defective. 

Familiar  Facts.  —  That  the  sounding-board 
of  a  piano  vibrates  while  the  instrument  is  being 
played,  may  be  seen  if  a  pin  or  other  small  body 
be  placed  on  it.  Blowing  into  a  pipe  sets  the  air 


SOUND.  87 

vibrating.  In  windy  weather  the  church-bells  of 
a  city  may  be  heard  farther  off  than  usual,  at  a 
place  which  lies  in  the  direction  of  the  wind; 
while  at  a  place  nearer  by,  but  in  an  opposite  di- 
rection, they  may  not  be  heard  at  all. 

Sound  is  caused  by  the  vibratory  motion  of  a 
sounding  body. 

If  a  cannon  is  fired  off  at  a  distance  of  about 
1100  feet,  the  flash  is  seen  instantaneously,  but 
the  report  will  be  heard  a  second  later.  At  twice 
that  distance,  the  report  will  be  heard  two  sec- 
onds later.  From  the  time  which  elapses  between 
the  flash  of  a  gun  on  a  vessel  in  distress,  and 
the  hearing  of  the  report  on  the  shore,  the  dis- 
tance of  the  vessel  may  be  found.  Thus,  if  ten 
seconds  have  elapsed,  the  vessel  is  about  11,000 
feet,  a  little  over  two  miles,  distant.  The  distance 
of  a  thunderstorm  may  be  ascertained  in  a  like 
manner,  by  counting  the  seconds  that  elapse  be- 
tween the  lightning  and  the  thunder  following  it. 

Sound  moves  at  the  rate  of  about  1100  feet  a 
second. 

Question. — 1  What  causes  the  noise  when  a 
piece  of  paper  is  torn  ?  2.  What,  when  a  piece 
of  wood  is  broken?  3.  What,  when  a  whip  is 
cracked  ? 

Bead  "Wonders  of  Acoustics,"  in  Illustrated  Library  of  Wonders. 
Bead  "The  Ear,"  in  "Human  Body"— Illust.   Library  of  Wonders. 
"Bead  "Sound  and  Echoes,"  p.  268,  in  Things  not  Generally  Known. 


88  FIRST  LESSONS   IN   PHYSICS. 


LESSON    XXIV. 

EVAPORATION — FOG— CLOUDS— RAIN— SNOW 
HAIL — DEW — FROST. 

Water  is  one  of  the  most  necessary  elements  in 
human  life.  By  the  Hindoos  and  other  pagan 
nations  it  was  revered  as  a  Deity  ;  and  the  masses 
of  bleached  bones  lying  around  the  few  wells  in 
the  desert  show  that  during  great  heat  the  want  of 
water  may  be  fatal  to  the  traveling  caravans. 

Familiar  Facts.  —  Moisture  on  a  slate  or  on  a 
piece  of  paper  will  disappear  very  soon.  Water  in 
a  tumbler,  exposed  to  the  air,  constantly  dimin- 
ishes, until  finally  none  is  left.  The  water  in 
streets,  cisterns,  ponds,  and  brooks  gradually  dis- 
appears. When  water  thus  passes  off  into  the  air, 
we  say  that  it  evaporates.  Evaporation  takes  place 
only  at  the  surface  of  liquids. 

By  evaporation,  water  is  changed  into  water 
vapor  (or  aqueous  vapor). 

Familiar  Facts. — In  summer  our  breath  is  in- 
visible ;  not  so  in  winter,  because  it  condenses 
immediately  after  leaving  the  mouth.  In  warm 
weather  the  vapors  rising  from  rivers,  swamps  and 
lakes  are  invisible.  There  may  be  a  great  quantity 
of  vapor  in  the  atmosphere,  and  yet  the  vapor  not 


FOG — CLOUDS— BAIN.  89 

be  seen.  When  the  air  near  the  earth  is  cool,  the 
vapor  becomes  visible,  and  then  we  call  it  Mist  or 
Fog.  Aqueous  vapor  coming  in  contact  with  cool 
air,  forms  Fog. 

The  vapor  may  not  be  perceived  below,  but  be- 
come visible  higher  up  in  the  atmosphere.  This 
takes  place  especially  when  the  warm,  moist  winds 
(south  or  southwest  winds)  come  in  contact  with 
colder  (north  or  northeast)  winds.  The  vapor  then 
forms  clouds. 

Fog  is  clouds  near  the  earth.  Clouds  are  fog  in  the 
upper  regions  of  the  air. 

Familiar  Facts. — A  piece  of  chalk,  a  piece  of 
earth,  a  lump  of  coal,  drop  quickly  ;  but  dust,  soot, 
and  finely  powdered  chalk,  descend  very  slowly. 
So  the  minute  particles  of  which  clouds  and  fog  are 
composed  may  float  in  the  air  for  a  length  of  time, 
because,  in  this  state,  their  downward  passage  is 
resisted  by  the  air.  Remember  that  soap-bubbles 
may  do  the  same.  But  when  aqueous  vapor  comes 
in  contact  with  cold  air,  its  minute  particles  unite — 
form  drops,  and  descend  as  rain.  On  their  passage 
through  the  air,  these  drops,  small  at  first,  increase 
in  size,  because  they  meet  with  more  aqueous  vapor 
in  the  air,  which  condenses  upon  them.  The  higher 
up  the  clouds,  the  greater  the  rain-drops.  (Why  ?) 
Rain  is  condensed  aqueous  vapor.  In  winter,  the 


90  FIKST   LESSONS   IN   PHYSICS. 

aqueous  vapor  in  the  atmosphere,  when  it  con- 
denses, freezes  and  forms  minute  crystals.  These 
increase  in  size  on  their  passage  through  the  air, 
Because  more  of  the  frozen  vapor  settles  upon  them, 
and  reach  us  as  snow- flakes.  Snow  is  frozen 
aqueous  vapor. 

On  stormy  summer-days,  stones  of  ice  sometimes 
fall  from  dense  clouds,  having  an  opaque  kernel 
and  a  transparent  rind.  They  may  be  disastrous 
to  green-houses  and  to  the  crops.  They  are  called 
Hail-stones.  But  it  is  not  known  why,  in  summer, 
such  cold  can  be  produced  as  to  form  solid  masses 
of  ice  in  the  atmosphere. 

Familiar  Facts.— Inhabited  rooms  contain  much 
aqueous  vapor.  A  part  of  it  is  exhaled  from  our 
lungs.  If,  in  summer,  a  tumbler  is  filled  with  cold 
water,  it  becomes  cold ;  the  aqueous  vapor  in  the 
air  arounl  it  cools  off,  condenses,  ,and  forms 
drops  of  water  all  over  the  glass.  If,  in  winter,  a 
cold  tumbler  is  brought  into  a  warm  room,  the 
vapor  around  the  glass  condenses,  and  forms,  like- 
wise, moisture  on  the  glass.  Axes,  iron  safes  and 
soda  fountains  are  vulgarly  said  to  "  sweat."  Moist- 
ure is  deposited  when  a  person  breathes  against  a 
cold  window-pane.  The  aqueous  vapor  of  heated 
apartments  condenses  on  cold  window-panes  and 
may  run  down  as  water. 

Aqueous  vapor  is  condensed  into  water  when  in  con- 
tact with  oodies  sufficiently  cold. 


DEW — FROST.  91 

Familiar  facts.— The    glistening    dew-drops 

which  you  have  so  often  admired  in  the  early 
morning-sun,  originate  in  the  same  manner.  In 
clear  weather,  the  objects  on  the  ground  cool  off 
during  the  night ;  and  at  the  same  time  the  aque- 
ous vapor  in  the  air  about  them  is  condensed. 
Grass  and  leaves,  in  general  all  pointed  objects, 
cool  more  quickly,  hence  they  have  the  most  dew. 
If  the  sky  is  cloudy,  the  clouds  act  like  a  screen ; 
they  throw  the  heat  back  to  the  earth.  Then  the 
objects  do  not  become  sufficiently  cold  and  no  dew 
is  formed.  Sometimes  there  is  no  dew,  and  yet 
the  sky  is  serene ;  this  is  owing  to  winds,  which 
bring  warmer  air  to  the  objects  so  that  they  can 
not  cool  off  sufficiently.  As  rain  is  aqueous  va- 
por condensed  in  the  air,  so  Dew  is  aqueous  vapor 
condensed  on  solid  bodies.  If,  during  the  night, 
objects  cool  off  to  a  greater  extent,  the  dew  which 
is  formed,  freezes.  Then  we  call  it  Frost. 
Frost  is  frozen  dew. 

Bead  "Lakes,  Springs,  Rain,  Dew,  Ice,"  in  "The  Earth  and  its 
Wonders." 

Bead  "Atmosphere,  Ocean,  Rivers,  Waterfalls,  in  "The  Sublime  in 
Nature." — Illustrated  Library  of  Wonders. 

Bead  "Dew  and  Water-vapor,"  in  "The  Phenomena  and  Laws  of 
Heat." — Illustrated  Library  of  Wonders. 


92  FIRST  LESSONS  IN  PHYSICS. 

LESSON    XXV. 

HEAT. — CONDUCTION  OF  HEAT. 

47.  EXPERIMENT. — Strike  a  piece  of  flint  and 

steel  together ;  sparks  will  fly  off. 

Familiar  Facts.—  On  a  stone  pavement,  at 
dusk,  sparks  may  be  seen  when  we  are  walking, 
or  when  a  horse  is  galloping.  In  these  cases, 
iron  (the  nails)  has  forcibly  struck  against  stone. 
The  sparks  which  we  see,  are  minute  particles  of 
iron,  or  steel,  which  have  been  heated  to  redness 
by  friction,  or  percussion. 

48.  EXPERIMENT. — Rub  a  key,  or  a  copper  coin, 
on  the  floor.    It  will  soon  become  heated. 

49.  EXPERIMENT. — Try  to  ignite  a  match  by 
rubbing  one  gently  on  a  piece  of  smooth  glass. 
It  will  not  burn,  because  there  is  insufficient  fric- 
tion; it  merely  glides  over  the  smooth  surface. 
But  if  rubbed  against  a  rough  surface,  such  as 
the  floor  or  a  brick,  the  match  presses  against  the 
projecting  parts  of  the  rough  surface  and,  owing 
to  the  friction  thus  produced,  it  becomes  heated 
and  ignites. 

Familiar  Facts.— "Wagon  wheels  have  so  much 
friction  at  their  axles,  that  unless  properly 
greased,  they  may  be  set  on  fire.  He  that  lets  him- 
self down  by  a  rope  has  his  hands  blistered.  On 
a  cold  day,  we  sometimes  rub  our  hands  together. 


HEAT. — CONDUCTION  OF   HEAT.  93 

Saws  and  augurs,  after  being  used,  feel  hot;  a 
piece  of  India-rubber,  warm.  This  shows  that 
Friction  produces  heat.  It  shows,  also,  that  Mo- 
tion may  be  converted  into  heat ;  for  friction  is 
motion  arrested. 

Familiar  Facts.— "By  holding  our  hands  near 
to  a  heated  stove  they  become  warm.  Heat  of 
the  stove  passes  first  to  those  parts  of  the  hands 
nearest  the  stove,  then  it  gradually  passes  to 
the  parts  next ;  and  so  on,  until  all  the  parts  of 
the  hand  are  heated. 

50.  EXPERIMENT. — Hold  a  short  wire  in  the 
flame  of  a  burning  lamp.    It  will  be  felt,  that 
even  the  part  of  the  wire  which  is  not  in  the 
flame,  is  heated ;  and  that  the  heat  increases  so 
that  we  must   soon  drop  the  wire.     It  is  plain 
that  the  heat  of  the  flame  was  imparted  first  to 
one  end  of  the  wire,  and  that  it  was  communi- 
cated successively  to  the  remaining  parts  of  the 
wire.    This  shows  that  Heat  may  be  communi- 
cated by  passing  successively  from  any  part  of  a 
body  to  the  remaining  parts.     This  communica- 
tion is  called  Conduction  of  Heat. 

51.  EXPERIMENT. — Hold  a  taper,  a  straw,  or  a 
thread  in  the  flame.    It  may  burn  quite  near  your 
fingers  without  hurting  them. 

52.  EXPERIMENT.— Take  up  the  wire  again 

(50  Exp.),  but  wrap  a  strip  of  paper,  or  cloth, 
around  the  end  in  the  hand.  If  held  in  the 


94  FIEST   LESSONS   IN   PHYSICS. 

flame  again,  there  is  scarcely  any  heat  felt.  Tea- 
pots and  soldering-irons  have  usually  wooden 
handles.  Why  ? 

Metals  are  good  conductors  of  heat.  Paper, 
wood,  cotton,  wool,  fur,  feathers,  ashes,  snow,  ice, 
straw,  and  air,  are  bad  conductors  of  Jieat. 

53  EXPERIMENT. — Place  a  wire  and  a  piece  of 
wood  upon  a  heated  stove,  and  let  them  remain 
there  for  a  while.  Both  obtain  the  same  temper- 
ature ;  yet,  if  touched  with  the  hand,  the  wire 
seems  to  be  the  warmer.  This  is  owing  to  the  fact 
that,  being  a  good  conductor,  it  instantly  imparts 
all  its  heat  to  the  hand.  If  you  touch  a  cold  iron 
bar,  it  instantly  takes  heat  from  the  hand,  and, 
therefore,  seems  cold. 

Questions. — 1.  Why  may  ice  be  kept  as  well 
in  a  feather  bed  as  in  an  ice-chest? 

2.  Why  do  mittens  keep  the  hands  warmer  than 
gloves  with  fingers  ? 

3.  Why  does  snow  melt  more   readily   on   a 
plank  than  on  a  rock  ? 

4.  Why  are  steam-chests  and  steam-cylinders 
often  covered  with  wood  ? 

5.  Why  are  the  walls  of  safes  often  filled  with 
fine  ashes  ? 

6.  Why  do  wide  garments  keep  us  warmer  than 
tight  ones  ? 

7.  Why  are  frame  houses  warmer  than  stone 
houses  ? 


CONDUCTION   OF  HEAT.  95 

Application  of  Conducting  Substances. 

/.  Good  Conductors. — They  conduct  heat  very  rapidly,  and,  therefore,, 
they  are  applied  in  order  to  diffuse  heat  quickly.  Thus,  to  boil  water 
and  roast  meat,  iron  vessels  are  used.  Iron  stoves  are  heated  in  very 
little  time. 

//.  Bad  Conductors. — They  conduct  heat  very  slowly,  but  they  also 
part  with  it  slowly ;  for  this  reason  we  apply  them  to  retain  heat. 
They  serve  to  prevent  a  warm  body  from  cooling  off,  and  a  cold  body 
from  becoming  heated. 

Familiar  Facts. — If  we  wish  to  warm  a  tumbler  on  a  heated  stove,, 
a  piece  of  paper  should  be  placed  between  the  gla?s  and  stove ;  other- 
wise the  glass  may  crack.  In  winter,  pieces  of  heated  wood  are  laid  in 
sleighs  to  keep  the  feet  warm.  Boards  are  placed  on  pavements,  and 
horsemen  like  to  have  wooden  stirrups,  because  wood  does  not  withdraw 
the  warmth  from  the  foot. 

Cotton  quilts,  woolen  garments,  blankets  and  furs  keep  the  body 
warm  in  winter  ;  they  neither  allow  the  warm  air  surrounding  the  body- 
to  pass  off,  nor  do  they  permit  the  cold  external  air  to  enter.  In  cold 
countries  animals  have  very  thick  fur;  some  in  our  latitude  have  thicker 
fur  in  winter  than  in  summer.  Northern  birds  have  thick  feathers. 
Feather  beds  are  in  favor  with  persons  fond  of  sleeping  very  warm. 
Blast-furnaces  are  sometimes  provided  with  double  walls,  and  the  space 
between  is  filled  with  ashes.  A  cover  of  snow  retains  the  heat  of  the 
earth;  thus  it  protects  the  winter  grain  from  the  cold.  The  Esquimaux  , 
build  themselves  huts  of  snow  and  ice.  Tender  trees,  vines  and  pumps- 
are  covered  with  straw  in  winter  to  protect  them  against  the  cold.  Ice- 
houses are  thatched  with  straw,  and  their  walls  filled  with  saw-dust,  to- 
prevent  heat  from  entering. 

Bead  "ffeat,"by  J.  Abbott.     Harper  &  Brother. 
Bead  "  Sources  of  Heat,"  in  The  Phenomena  and  Laws  of  Heat. 
Read  "  Good  and  Bad  Conductors"  in  The  Phen.  and  Laws  of  Heat. 
Read  "  Woolen  Clothing >" 'p.  296,  in  Things  not  Generally  Known. 


96  FIRST  LESSONS   IN   PHYSICS. 


LESSON    XXVI. 

DKAUGHT. 

54.  EXPERIMENT. — Shreds  of  cotton,  or  small 
strips  of  paper,  held  over  a  heated  stove  or  regis- 
ter, or  over  a  lamp  flame,  will  move  upward,  and, 
if  let  go,  they  will  ascend.     The  air  above  the 
source  of  heat  is  heated.   From  the  fact  that  boil- 
ing water  runs  over,  and  from  a  great  many  other 
facts  (Less.  XXVII),  we  know  that  heat  expands 
"bodies,  and  that  heated  air  is  expanded,  and  thus 
takes  up  more  space  than  before,  and,  therefore, 
has  less  specific  gravity  (Less.  II)  than  it  had 
when  cold.    Now,  as  air  rises  in  bubbles  through 
water,  so  does    heated  air  ascend   in  currents 
through  the  colder  air. 

55.  EXPERIMENT.— Insert  one  end  of  a  rod 

upright  in  a '  cork,  and  stand  the  whole  on  a 
heated  stove  or  register.  Suspend  from  the  top  a 
band  of  paper,  cut  in  the  shape  of  a  spiral ,  the 
upward  current  of  hot  air  will  cause  it  to  revolve. 

56.  EXPERIMENT. — Bring  a  thermometer  near 
the  floor  of  a  room ;  then,  near  the  ceiling.    It  will 
be  seen  that  near  the  ceiling  the  air  is  warmer 
than  below.    Heated  air  rises. 

Why  do  balloons,  smoke  and  steam  rise  ?   (See 
Lesson  II.) 


DRAUGHT.  97 

57.  EXPERIMENT.— If  a  window  in  a  heated 
room  be  opened  above  and  below,  the  flame  of  a 
burning  candle,  held  in  the  opening  above,  will 
be  blown  from  the  room  ;  if  held  in  the  opening 
below,  into  the  room. 

Familiar  Facts.— The  same  may  be  observed 
with  cotton  shreds  in  place  of  the  flame.  This 
shows  that  the  colder  air  from  out- doors  rushes 
into  the  room  from  below,  while  the  heated  air  of 
the  room  flows  out  above.  The  colder  air  is  con- 
fined to  the  lower  parts  of  a  room,  because  it 
has  greater  specific  gravity  than  heated  air. 
Wherever  a  fire  is  burning,  a  current  of  air,  or 
draught,  is  produced.  A  draught  is  also  noticed 
when  passing  from  the  sun  into  the  shade,  for 
where  the  sun  shines,  warmer  air  ascends,  and  is 
replaced  by  the  colder  air  from  below.  Chimneys 
serve  to  increase  the  draught,  because  they  en- 
close a  tall  column  of  heated  air,  which  has  less 
specific  gravity  than  the  outer,  colder  air.  The 
latter  presses  in  with  increased  force  proportion- 
ate to  the  height  of  the  chimney.  If  a  handker- 
chief be  tied  around  the  small  openings  under 
the  burner  of  a  lighted  lamp,  the  flame  will  be 
extinguished.  The  same  happens,  also,  if  the  top 
of  the  chimney  is  covered  with  a  piece  of  glass ; 
in  this  case  the  draught  is  stopped  because  the 
heated  air  can  not  pass  out,  and  consequently  no 
fresh  air  come  in. 
7 


98  FIRST   LESSONS   IN   PHYSICS. 

Heated  air  rises ;  colder  air  flows  in  to  take  its 
place. 

Familiar  Facts. — Near   heated  ground,  the 

air  ascends  and  is  replaced  by  colder  air.  This 
causes  our  atmosphere  to  be  in  constant  motion. 
The  currents  thus  produced  are  called  Winds. 

Application. — Chimneys  (in  lamps,  stores,  fac- 
tories, &c.,  &c.)  Ventilation  of  rooms  and  halls. 

Bead  "Draught  and  Ventilation,"  p.  269,  in  Things  not  Generally 
Known. 

Bead  "  Winds  and  Currents"  p.  279,  in  Things  not  Generally  Known. 

Read  "Does  the  Sun  Influence  a  Fire"  p.  267,  in  Things  not  Gen- 
erally Known. 


REVIEW. 

LESSON  xxiv. — 

1.  Heat  changes  liquids  into  Vapors.     Vapor  of 

water  is  called  Aqueous  Vapor.  The  process 
is  called  Evaporation. 

2.  Aqueous  vapor  coming  in  contact  with  cool 

air,  forms  Fog.  Fog  is  clouds  near  the  earth. 
Clouds  are  fog  in  the  higher  regions  of  air. 

3.  Aqueous  vapor,  in  contact  with  cool  air,  forms 

Fog  ;  in  contact  with  cold  air,  Rain;  with 
cold,  solid  bodies,  Dew  ;  with  intensely  cold 
air,  Snow.  Frost  is  frozen  dew. 


EXPANSION  BY   HEAT.  99 


LESSON    XXVII. 

EXPANSION    BY  HEAT. — THERMOMETER. 

58.   EXPERIMENT.  —  Heat  a  fine  glass  tube, 

closed  at  one  end,  and  partly  filled  with  water  ;  the 
water  will  be  seen  to  rise  as  it  becomes  heated. 
Warm  water  takes  up  a  larger  space  than  cold. 

Familiar  Facts.  —  A  cold  tumbler  placed  on  a 
heated  stove  will  crack  at  the  bottom.  As  it  gets 
hotter  below  than  above,  it  suddenly  expands  below 
more  than  it  does  above,  and  so  the  tumbler  must 
break.  How  may  it  be  prevented  from  cracking  ? 
(Lesson  XXY,  p.  95.)  A  bladder,  filled  with  air 
and  tied  up  at  the  end,  expands  if  near  a  hot  stove 
or  register.  The  air  inside  becomes  heated,  and 
heated  air  takes  up  a  greater  space  than  cold  air. 
A  flask  with  a  ground  glass- stopper  is  sometimes 
difficult  to  open  ;  if  it  be  gently  heated  around  the 
neck  the  stopper  may  be  taken  out  without  dif- 
ficulty. The  rails  on  a  railroad  track  are  laid  so 
that  their  ends  shall  be  at  a  slight  distance  from 
each  other ;  in  summer  their  ends  are  very  nearly 
together;  in  winter  they  are  farther  apart.  Tires 
are  heated,  nearly  red-hot  before  they  are  placed  on 
carriage-wheels,  for  they  are  then  wider,  and,  on 
cooling,  fit  tight  to  the  wheels. 

Heat  expands  all  bodies. 


t 
100  FIRST   LESSONS    IN   PHYSICS. 

Temperature. — A  substance  is  said  to  cool  when 
it  parts  with  sensible  heat,  that  is,  with  such  heat 
as  may  be  felt.  In  the  previous  instance  the  tires 
lost  most  of  their  sensible  heat.  The  amount  of 
sensible  heat  which  a  body  has,  is  its  temperature. 


Let  us  now  consider  the  Thermometer.  The 
silvery  substance  in  it  is  one  of  the  few  elements 
having  the  liquid  state  at  ordinary  temperature ;  at 
an  intense  degree  of  cold — such  as  Arctic  explorers 
experience — it  freezes  into  a  solid  mass.  Its  name 
is  Mercury,  or  Quicksilver.  If  the  mercury  is  heated, 
it  expands  ;  it  rises  in  the  tube,  simply  because  it 
has  no  other  place  to  which  to  go.  On  cooling, 
it  contracts,  and  falls.  It  may  be  heated  by  the  at- 
mosphere, that  is,  by  the  sun  ;  or  by  hot  water  ;  by 
steam;  by  heated  oil,  or  by  the  natural  warmth 
of  the  hand  when  placed  upon  it.  On  examining 
the  thermometer,  you  notice  that  it  consists  of  a 
glass  tube  with  a  bulb  below.  Both  tube  and  bulb 
are  closed.  The  bulb  and  a  portion  of  the  tube  are 
filled  with  mercury.  Above  the  mercury  is  a  vac- 
uum. The  vacuum  is  obtained  by  heating  the 
mercury  to  a  very  high  degree ;  its  vapors  then  fill 
the  tube,  the  open  end  of  which  is  now  fused  ;  this 
closes  the  tube.  The  whole  is  now  exposed  to  cold ; 
this  condenses  the  mercury-vapors  into  liquid  mer- 


THERMOMETER.  ,,„  ,101 

cury,  leaving  a  vacuum  behind.  The  frame  is 
not  an  essential  part  of  the  thermometer.  A 
little  above  the  bulb  is  a  point,  marked  Freez- 
ing Point  Everywhere  on  the  earth,  ice  melts  at 
the  same  degree  of  temperature.  So,  after  the 
tube  is  sealed  and  cooled  off,  it  is  placed  in  melt- 
ing ice.  Immediately  the  mercury  sinks,  because 
the  cold  contracts  it.  It  occupies  now  a  mucb 
smaller  space,  and  when  it  has  settled,  its  low- 
est point  is  carefully  marked,  either  on  the 
frame  or  by  etching  it  on  the  glass  tube.  This 
point  is  called  the  "  Freezing  Point."  It  has  also 
been  found  that  all  over  the  earth,  water,  in  low 
countries,  boils  at  the  same  temperature.  So  the 
thermometer  is  now  held  upright  in  the  hottest 
steam  issuing  from  boiling  water.  Heat  expands 
all  bodies ;  hence  the  mercury  expands  and  is 
seen  to  rise  in  the  tube.  The  point  to  which  it 
ascends  is  carefully  marked ;  it  is  the  "Boiling 
Point."  The  space  between  the  two  points  has 
been  divided  into  degrees.  By  means  of  these 
degrees,  we  are  enabled  to  indicate  the  tempera- 
ture which  a  body  has  acquired. 

Read  "  Expansion — Thermometer,"  in  "The  Phenomena  and  Laws 
of  Heat.  - ' 


FIRST   LESSONS   IN   PHYSICS. 


LESSON    XXVIII. 

THERMOMETER  COMPARED  WITH  BAROMETER. 

59.  EXPERIMENT. — If  the  palm  of  the  hand, 

after  being  rubbed  a  little  so  as  to  be  perfectly 
dry,  is  held  to  the  thermometer-bulb,  the  mercury 
will  rise  to  a  point  which  marks  the  Blood-heat 
of  the  human  body.  It  happens  to  be  indicated 
on  our  thermometers  by  the  number  99.  This  is 
owing  to  the  fact,  that  in  our  country,  and  also  in 
England,  the  space  between  the  freezing  and  the 
boiling  points  is  measured  by  very  small  degrees, 
of  which  there  are  180  between  those  two  points. 
Fahrenheit,  a  philosophical  instrument  maker, 
divided  that  space  into  180  degrees.  He  com- 
menced counting,  however,  not  at  the  Freezing- 
point,  but  at  a  point  below,  which  is  the  zero 
point  of  his  scale  ;  this  brings  the  freezing  point  to 
32°;  the  boiling-point  is  marked  212°.  In  some 
European  countries  the  Freezing-point  is  marked 
0°  ;  the  Boiling-point  80°.  That  is,  the  space  be- 
tween the  two  points  is  divided  into  only  80 
degrees.  Each  degree  of  this  kind  is  much 
larger  (how  many  times  as  large  ?)  than  one  of 
the  former  kind,  the  Fahrenheit.  From  the 
name  of  the  French  philosopher  who  arranged 


THERMOMETER — BAROMETER.  103 

this  scale,  its  degrees  are  called  degrees  Reau- 
mur. Thus  80°  R.  is  equivalent  to  180°  F.  Far 
more  convenient  than  either  of  the  two  preceding 
scales  is  the  one  of  Celsius.  He  divided  the  space 
between  the  freezing  and  the  boiling  points  into 
100  degrees.  The  use  of  this  division  is  gradually 
spreading.  According  to  it,  100°  a=l8Q°F= 80°^. 

The  minus  sign  distinguishes  numbers  below  0. 

Centigrade.  Fahrenheit. 

Boiling-point 100° - 2 12° 

50°  122° 

25°  77° 

Freezing-point Op 32° 

—17JQ  0° 

—  50°  —  58° 

The  healthiest  temperature  for  any  room  is 
about  65°^.  Our  rooms  should  not  be  heated 
beyond  that  in  winter.  Thermometers  should  be 
placed  at  equal  distance  from  stove,  or  lireplace, 
and  the  windows,  so  as  to  show  the  mean  tem- 
perature of  the  air. 

Questions. — If  in  New  York  the  mercury  stands 
at  80°  above  zero,  how  would  the  same  tem- 
perature be  indicated  in  Paris  (according  to  O. 
degrees)?  How  in  Berlin  (according  to  O.  de- 
grees) ?  By  what  numbers  would  the  blood-heat 
point  be  indicated  according  to  those  scales  ?  By 
what  number  is  the  point  of  healthiest  tempera- 
ture indicated  in  O.  degrees  ?  (See  p.  174.) 


104  FIRST   LESSONS   IN   PHYSICS. 

Thermometer  and  Barometer  Compared. 

Four  points  in  common  : 

1.  Both  instruments  consist  of  a  glass  tube. 

2.  Both  have  mercury  in  their  tube. 

3.  Both  have  a  vacuum. 

4.  Both  have  a  graduated  scale. 
Four  points  of  difference : 

1.  The  thermometer -tube  is  closed  above  and 

below  ; 

the  barometer  tube  is  closed  above  but  open 
below,  so  that  the  pressure  of  air  may  reach 
the  mercury  within  it. 

2.  In  the  thermometer-tube,  the  mercury  rises 

and  falls  on  account  of  the  effects  of  heat 
and  cold  ; 

in  the  barometer- tube,  the  mercury  rises  and 
falls  on  account  of  the  increase  or  decrease 
of  air-pressure. 

3.  The  thermometer    has   a   scale   of    degrees 

whose  size  is  arbitrary  and  may  be  differ- 
ent in  different  thermometers ; 
the  barometer  has  a  scale  of  inches,  and  frac- 
tions of  inches ;  its  scale  is  of  less  extent, 
and  only  at  the  upper  part  of  the  tube. 

4.  Mercurial     Thermometers    may    have    any 

length; 
mercurial  Barometers  have  uniform  length. 


ATMOSPHERIC  ENGINE.  105 


LESSON    XXIX. 

THE   ATMOSPHEEIO   ENGINE. 

1.  If  we  look  at  a  sewing-machine  while  it  is 
in  motion,  our  attention  is  immediately  called  to 
a  long,  upright  rod,  made  to  move  up  and  down 
by  the  stroke  of  the  foot.   The  rod  being  fastened 
to  a  wheel,  it  is  evidently  its  up  and  down  motion 
that  causes  the  motion  of  the  wheel  and  with  it, 
that  of  the  machine.    You  need  but  fasten  a  rod 
to  the  edge  of  a  toy-wheel,  and  you  may  demon- 
strate the  same.    Motion  in  a  straight  line — rec- 
tilinear motion — is  thus  converted  into  circular 
motion. 

2.  This  was  known  thousands  of  years  ago; 
but,  strange  to  say,  the  principle  upon  which  the 
steam-engine  is  founded, was  not  thought  of  until 
about  1690,  A.  D.     At  that  time,  Professor  Papin, 
an  exiled  Frenchman  living  in  Germany,  pub- 
lished a  little  work,  in  which  he  says :   "  There  is 
a  property  peculiar  to  water,  owing  to  which  a 
small  quantity  of  that  liquid,  if  heated  and  con- 
verted into    steam,  acquires  a  force  of  elasticity 
which  much  resembles  that  of  air.     When  cooled 
down,  it  returns  to  the  liquid  state,  and  loses  its 
elasticity.        I  am,  therefore,  inclined  to  believe 


106  FIRST   LESSONS  IN   PHYSICS. 

that  machines    may  be   constructed  which  are 
moved  by  the  application  of  heat  to  water." 

3.  These    words   laid    the   foundation  for  the 
greatest  change  which  human  society  ever  ex- 
perienced.  The  machine  that  effected  this  change 
has  benefited  humanity  more  than  all  the  gold 
mines  in  the  world.     The  steam-engine  not  only 
reveals  to  us  the  hidden  treasures  of  the  earth ; 
"  it  can  engrave  a  seal ;  crush  masses  of  obdurate 
metal  like  wax  before  it ;  draw  out,  without  break- 
ing, a  thread  as  fine  as  a  gossamer,  and  lift  a  ship 
of  war  like  a  bauble  in  the  air.     It  can  embroider 
muslin  and  forge  anchors ;  cut  steel  into  ribands 
and  impel  loaded  vessels  against  the  fury  of  the 
winds  and  waves."    And  when  it  flies  with  the 
rapidity  of  a  bird,  over  land  and  water,  hurling 
dense 'masses  of  steam  and  smoke  into  the  air, 
does  it  not  look  like  some  gigantic  monster  that 
contains  the  strength  and  the  power  of  thousands 
of  men  ?    Well  may  we  admire  the  genius  of  man 
that  can  turn  one  of  Nature's  simplest  forces  to 
such  wonderful  account. 

4.  The  simplicity  of  Papin's  statement  is  demon- 
strated by  his  own  application.      Knowing  that 
steam  was  elastic. like  air  (Lesson  X),  he  immedi- 
ately proceeded  to  the  construction  of  an  appar- 
atus which,  although  its  practical  usefulness  was 
impeded   by  its   slowness,  was  the  first  steam' 
engine  ever  built. 


THE   ATMOSPHERIC   ENGINE.  107 

60,  EXPERIMENT. — Papin's  apparatus  may  be 

illustrated  by  a  test-tube  (one  of  tin  is  preferable 
inasmuch  as  glass  breaks  easily,)  as  shown 
in  Fig.  22.  A  small  disk  of  wood,  with  a 
packing  of  thread  around  it  to  make  it  fit 
tight,  is  made  into  a  piston,  P,  moving  in  a 
tube  nearly  air-tight,  and  attached  to  a  rod. 
The  tube  is  then  filled  with  water  about 
an  inch  high,  which  is  made  to  boil  over  a 
flame  after  the  piston  is  carefully  placed 
in  the  tube.  The  generation  of  steam  causes 
the  piston  to  rise.  E  is  an  outlet  for 
surplus  steam,  and  made  in  the  upper  part 
of  the  tube.  Between  the  water,  when  boiling, 
and  the  piston  there  is  no  air ;  the  space  is  filled 
with  steam.  On  immersing  the  tube  in  cold  water, 
the  rod  descends  again,  because  the  steam  below 
.the  piston  is  condensed  by  the  cold,  and  because 
a  vacuum  is  thus  formed  between  the  piston  and 
the  surface  of  the  heated  water.  What  is  it  that 
forced  the  piston  down  ?  The  answer  is :  "  At- 
mospheric Pressure."  (Lesson  XI.) 

5.  In  place  of  the  small  tube  of  this  experiment, 
Papin  used  a  large  iron  cylinder,  with  proper 
piston  and  piston-rod.  We  can  readily  imagine 
how,  by  throwing,  at  regular  intervals,  a  stream 
of  cold  water  on  the  cylinder,  he  produced  an 
up-and-down  motion  of  the  rod ;  and  how  the  ma- 
chine must  needs  have  been  slow — too  slow  to  be 


108  FIRST  LESSONS  IN    PHYSICS. 

practically  applied.  A  steam-engine  built  upon 
the  principle  of  Papin's — that  is,  one  not  worked 
by  the  expansive  force  of  steam,  but  merely  by 
atmospheric  pressure — is  not  a  steam-engine.  It 
is  an  "Atmospheric  Engine." 

6.  Captain  Savery,  an  Englishman,  constructed 
at  about  the  same  time,  an  apparatus  in  which 
the  steam  served  the  purpose  of   raising  water. 
The  steam  was  generated  in  a  separate   boiler, 
and  thence  led  into  a  chamber  where  it  was  con- 
densed by  cold  water  flowing  over  the  chamber. 
The  apparatus,  however,  was  very  imperfect,  and 
used  only  for  pumping  water.     Still,    his   was 
the  merit  of  having  constructed  the  first  Atmos- 
pheric Engine   that  ever  received  practical   ap< 
'plication. 

7.  Thomas  Newcomen,  a  hardware  man,  and 
John  Cowley,  a  glazier,  both  Englishmen,  by  their 
brilliant  invention,  completely  eclipsed   Savery's 
engine.     They  improved  upon  Papin's   plan  in 
this,  that  they  generated  the  steam  in  a  boiler — 
not  in  the  cylinder — and  that  they   condensed 
it,  not  by  cooling  the  boiler  from  without,  but 
by  forcing  a  jet  of  cold  water  into   the   steam. 
The  machine  was  put  to  immediate  use  in  the 
coal-mines  of  England;  and  it  is  sometimes  used 
even  at  present,  in  places  where  a  great  mass  of 
water  is  to  be  pumped  out.     Its  construction  is 
very  simple. 


THE   ATMOSPHERIC   ENGINE. 


109 


8.  It  consists  of  the  boiler,  A  (Fig.  23),  where  the 
steam  is  generated,  and  the  cylinder,  Z?,  which  is 


PIG,  23. 

connected  with  the  boiler  by  means  of  the  pipe, 
F.  When  steam  has  entered  the  cylinder,  and 
the  piston  O  is  raised,  the  stop-cock,  a,  is  closed. 
This  shuts  off  the  connection  between  the  boiler 
and  the  cylinder.  The  stop-cock,  &,  is  then 
opened,  and  a  jet  of  cold  water  from  the  small 
reservoir,  O,  is  thrown  into  the  cylinder.  This 


110  FIEST   LESSONS  IN   PHYSIOS. 

condenses  the  steam  in  the  cylinder ;  a  vacuum 
is  formed  below  the  piston,  and  atmospheric 
pressure  forces  the  piston  down.  The  water 
from  the  condensed  steam  flows  off  through  the 
pipe,  d,  into  a  reservoir  with  water.  (At  the 
end  of  d  is  a  valve  opening  outward.)  By  means 
of  an  iron  chain,  the  piston-rod,  H,  is  attached  to 
a  working-beam,  which  swings  on  the  pivot,  Z>, 
and  which  is  connected  at  the  other  end  with  the 
rod  E.  This  rod  is  raised  when  the  piston  de- 
scends. When  the  stop-cock,  a,  is  opened  again, 
the  steam  rushes  again  into  the  cylinder ;  but  as 
the  force  of  pressure  of  the  steam  scarcely  ex- 
ceeds that  of  the  air  over  the  piston,  the  piston 
would  not  rise,  were  it  not  for  the  heavy  weight 
attached  to  the  rod,  E.  This  weight  falls  when- 
ever steam  is  let  in  under  the  piston,  O;  and  in 
falling,  forces  one  arm  of  the  working-beam 
down,  causing,  at  the  same  time,  the  piston  at 
the  other  arm  to  rise.  The  rod  P  is  the  piston- 
rod  of  a  pump,  and  is  fastened  to  the  weight. 

In  the  Atmospheric  ^Engine  the  piston  is  raised 
by  Gravity,  and  lowered  by  Atmospheric  Pressure. 

State  the  principal  points  of  Papin's  engine; 
of  Savery's ;  and  Newcomen's. 

Read  "H.  Potter,"  in  "Inventions  and  Discoveries,"  by  Tetuple. 
London:   Groombridge. 


STEAM-ENGINE.  Ill 


LESSON    XXX. 

THE   STEAM-ENGINE. 

1.  Half  a  century  had  passed  away.  New- 
comen's  engine  had  been  introduced  into  most  of 
the  coal-mines  of  England,  when,  in  the  winter  of 
1763,  a  young  mechanic,  James  Watt,  in  Glasgow, 
was  employed  by  the  University  of  that  city  to 
repair  one  of  Newcomen's  engines.  The  task 
which  this  man  of  uncommon  mind  was  about  to 
undertake,  marks  a  new  era  in  the  history  of 
steam-power,  an  era  that  finally  resulted  in  the 
perfection  of  a  machine  which  is  an  element  of 
modern  civilization.  On  trying  the  engine  after 
he  had  repaired  it,  young  Watt  perceived  that  it 
was  very  imperfect.  The  principal  defect  con- 
sisted in  this,  that  the  machine  used  a  great  deal 
more  steam  than  was  needed  for  the  motion  of 
the  piston.  For  when  the  stream  of  cold  water 
was  thrown  into  the  cylinder,  the  steam  was  con- 
densed ;  but  at  the  same  time,  the  cylinder  was 
cooled  down  to  such  an  extent,  that  when  fresh 
steam  was  admitted  again,  a  great  quantity  of  it 
was  wasted  in  reheating  the  cylinder ;  and  thus 
there  was  a  loss  of  money  in  direct  proportion  to 
the  amount  of  fuel  necessary  for  producing  the 
quantity  of  steam  equivalent  to  the  quantity 


112  FIRST   LESSONS   IN   PHYSICS. 

wasted.  On  calculating  the  loss,  it  was  found 
that  I  of  all  the  fuel  used  was  wasted ;  that  is, 
employed  in  reheating  the  cylinder.  The  question 
with  Watt  now  was,  How  can  the  cylinder,  in- 
stead of  being  cooled,  be  kept  permanently  hot  ? 
In  other  words,  How  can  the  steam  be  condensed 
without  at  -the  same  time  cooling  the  cylinder  ? 

2.  Watt's  genius  solved  the  problem  by  an  in- 
vention of  surprising  simplicity.    He  condensed 
the  steam  in  a  separate  chamber,  the  condenser. 
It  stood  in  a  chest  filled  with  water,  and  was  con- 
nected with  the  cylinder  by  means  of  a  pipe. 
Thus  the  steam  could  be  condensed  without  cool- 
ing the  cylinder,  by  simply  leading  it  off.     The 
immediate  result  was  the  saving  of  I  of  thejfuel. 

3.  But  Watt  did  not  stop  here.    He  noticed  that 
the  air  entering  the  heated  cylinder  as  the  piston 
went  down,  also  cooled  the  cylinder.     This  caused 
a  waste  of  steam,  as  the  cylinder,  in  order  not  to 
condense  the  fresh  steam  entering,  had  first  to  be 
reheated  to  212°.    To  remedy  this,  he  dispensed 
with  the  air  entirely,  in  providing  the  cylinder 
with  a  cover  pierced  in  the  center  so  as  to  admit 
the  piston-rod  air-tight.     The   air  (atmospheric 
pressure)  could  now  no  longer  act  upon  the  piston ; 
how  then  was  the  piston  to  descend?     It  was 
made  to  descend  by  allowing  steam  from  the 
boiler  to  enter  above  the  piston,  through  a  pipe 
connecting  the  boiler  with  the  upper  part  of  the 


STEAM-ENGINE.  113 

cylinder ;  and  to  pass  out  again  through  a  pipe 
connecting  the  cylinder  with  the  condenser. 
Thus  while  there  was  a  vacuum  established  in 
the  lower  part  of  the  piston,  steam  was  admitted 
into  the  upper  part ;  the  upper  part  then  being 
made  a  vacuum  by  leading  the  steam  off  into  the 
condenser,  fresh  steam  was  admitted  into  the 
lower  part  and  forced  the  piston  up.  By  this  im- 
provement, the  steam  not  only  served  as  a  ready 
means  for  obtaining  a  vacuum,  as  in  Newcomen's 
engine,  but  its  expansive  force  was  also  made  use 
of,  and  from  that  time  Watt's  engine  was  no 
longer  an  atmospheric,  but  a  steam  engine. 

4.  The  atmospheric  engine  was  "Single  Act- 
ing ; "  it  did  work  only  while  the  piston  descend- 
ed ;  the  rise  of  the  piston,  as  we  remember  from 
the  preceding  lesson,  was  effected  by  gravity. 
The  power  obtained  by  this  machine  was  so  small 
that  it  could  not  overcome  the  resistance  of  a 
wheel,  and,  therefore,  it   was   used  mainly  for 
pumping  water  out  of  coal-mines. 

5.  It  will  now  be  readily  understood,  that  by 
admitting  the  steam  alternately  above  and  below 
the  piston,  Watt  made  the  steam-engine  "Double 
Acting,"  and  this  was,  perhaps,  the  most  impor- 
tant of  all  his  improvements.     For  now,   rotary 
motion  could  be  produced,  without  which  no  lo- 
comotive or  steamboat    could  ever   have   been 
thought  of. 


114  FIEST  LESSONS    IN  PHYSICS. 

Watt  died  in  1819,  honored  and  admired  by  all 
who  knew  him.  Within  a  short  time  after  his 
death,  five  large  statues  were  erected  to  his 
memory. 


6.  In  all  the  engines  constructed  by  Watt,  the 
power  of  the  steam  was  low  ;  it  amounted  scarcely 
to  more  than  li  atmospheres  (1J  as  much  as  the 
pressure  of  our  atmosphere;  that  is  1J  times  15 
pounds  to  the  square  inch  of  surface).  The  alter- 
nate condensation  of  steam  on  either  side  of  the 
piston  was,  therefore,  the  only  means  of  obtaining 
the  up-and-down  motion  of  the  piston ;  for  the 
feeble  expansive  force  of  the  steam  was  totally 
insufficient  to  overcome  the  counter-pressure  of 
the  atmosphere.  But  by  employing  steam  of 
greater  expansive  force  —  that  is,  steam  capable 
of  exerting  a  greater  pressure,  it  was  found  that 
the  condenser  could  be  dispensed  with,  as  the 
pressure  of  this  steam  was  sufficiently  great  to 
move  the  piston  against  the  resistance  of  the 
atmosphere.  Engines  usually  having  a  steam-pres- 
sure of  from  3  to  15  atmospheres  (45  to  225  pounds 
of  pressure  to  the  square  inch),  and  which,  as  a 
rule,  have  no  condenser,  but  send  their  exhaust 
steam  directly  into  the  atmosphere,  are  called 
Higli  Pressure  Engines;  while  those  generally 
working  with  a  lower  pressure,  and  always  with  a 
condenser,  are  called  Low  Pressure  Engines. 


STEAM-ENGINE. 


115 


7.  The  admission  of  steam  into  the  cylinder 

is  now  accomplished  by  means  of  a  sliding-valve. 


Steam- Chest. 


CylindSr. 


It  is  enclosed  in  a  square  box,  called  the  steam- 
chest  (See  Fig.  24),  which  is  attached  to  one  side 
of  the  cylinder.  When  the  steam  from  the  boiler 
reaches  the  steam-chest  through  the  opening,  0, 
it  fills  the  chest  at  once,  and,  as  the  sliding-valve 
keeps  the  opening,  6,  closed,  it  presses  through 
the  opening  a  into  the  cylinder.  There  it  fills  the 
upper  part  and  forces  the  piston  down.  This  it 
does  because  a  vacuum  has  been  formed  on  the 
other  side  of  the  piston,  or,  as  is  the  case  in  High 
Pressure  Engines  (see  Fig.  above),  by  the  immense 
expansive  force  of  the  steam.  At  the  same  time, 
however,  the  sliding-valve  (which  rises  when  the 
piston-rod  descends,  and  descends  when  the  pis- 


116 


FIRST   LESSON'S   IN   PHYSICS. 


ton-rod  rises,)  has  moved  upward,  and  shuts  off 
the  steam  from  a  (see  Fig.  25) ;  the  steam  must 


Steam-Chest. 


Cjhnder 


now  enter  through  the  opening  b  and  force  the 
piston  up.  Meanwhile  the  old  steam  above  the 
piston  passes  through  a  and  e  into  a  tube  lead- 
ing to  the  condenser.  In  a  steam-eiigine  which 
has  no  condenser,  as,  for  example,  the  loco- 
motive, the  old  steam  passes  through  e  into  the 
air.  After  the  piston  has  arrived  above,  the 
process  is  renewed,  owing  to  the  sliding-valve 
having  a  motion  opposite  to  that  of  the  piston ; 
thus  steam  is  admitted  alternately  above  and  be- 
low the  piston  which,  as  mechanics  say,  moves  in 
a  vacuum,  or  rather,  in  a  space  filled  with  steam. 

In  figs.  24  and  25,  the  exhaust  e  is  too  narrow.  It  should  be  consider- 
ably wider  than  either  a  or  b. 


STEAM-ENGINE.  117 

8.  When  we  look  at  a  locomotive  rushing  past 
us  at  full  speed,  we  notice  a  horizontal  iron  rod 
moving  back  and  forth.     The  rod  connects  two 
large  wheels,  and  runs  at  one  end  in  a  wide  brass 
cylinder.    Next  to  this  cylinder  is  the  steam-chest, 
a  small  square  box.     It  is  in  the  cylinder  that 
the  motory  power  is  imparted  to  the  engine.    In 
addition  to  these  things,  we  see  a  great  many 
wheels,  pipes  and  rods ;  but  they  mostly  serve 
minor  purposes.     The  main  parts  of  the  locomo- 
tive are  the  steam -chest,  cylinder,  piston,  piston- 
rod,  the  large  wheels,  and  the  boiler. 

9.  The  steam,  by  means  of  the  sliding- valve, 
causes  the  back-and-forth  motion  of  the  piston  in 
the  cylinder ;  by  this,  it  causes  the  back-and-forth 
motion  of  the  piston-rod  ;  and  by  this,  the.  revolu- 
tion of  the  large  wheels.     The  wheels    roll   on 
the  track ;  they  cause  the  locomotive  to  move  on- 
ward, and  the  locomotive  pulls  the  cars  attached 
to  it. 

Read  "James  Watt,"  in  "Pursuit  of  Knowledge,"  Vol.  II.  New 
York:  Harper  &  Bros. 

Read  "The  Locomotive  Engine,"  by  C.  Colburn.    H.  C.  Baird,  Phila. 

Read  "The  Steam- Engine, "  \>y  David  Read.  Hurd  &  Houghton, 
New  York. 

Read  "The  Railway  and  its  Cradle"— "The  Youth  of  James 
Watt" — in  "Inventions  and  Discoveries."  Groombridge  &  Sons, 
London. 


118  FIRST   LESSONS   IN   PHYSICS. 


LESSON    XXXI. 

REVIEW. 

LESSON  xxni. — 

1.  The  motion  of  a  body  produces  vibrations  in 

the  air  which,  ir  they  impress  the  ear,  give  us 
the  sensation  of  sound.     Sound,  therefore,  is 
merely  the  effect  of  a  vibrating  motion  upon 
the  ear. 
LESSON  xxv. — 

2.  Heat  may  be  communicated  by  passing  suc- 

cessively from  one  part  of  a  body  to  the  other 
parts.  This  mode  of  communication  is  called 
Conduction  of  Heat. 

LESSON  xxvi. — 

3.  Heated  air  rises,  because  it  has  less  specific 

gravity  than    cold    air.      This    fact    causes 
Draught  and  Winds. 
LESSON  xxvn. — 

4.  All  bodies  are  expanded  by  heat;  and  con- 

tracted by  cold. 

5.  What  we  call  "  Heat,"  is  merely  a  vibrating 

motion  among  the  minute  invisible  parts 
(molecules)  of  a  heated  body.  We  can  not 
see  that  vibrating  motion,  but  we  can  feel  it. 

6.  What  we  call  "Sound,"  is  merely  a  vibrating 

motion  of  masses.  We  can  neither  see  nor 
feel  that  vibrating  motion,  but  we  can  hear  it. 


KEVIEW.  119 

7.  As  sound  is  the  effect  of  vibratory  motion  upon 

the  ear,  so  heat  is  the  effect  of  vibratory  mo- 
tion upon  the  nerves. 
LESSON  xxix. — 

8.  In  the  old  Atmospheric  Engine  the  piston  is 
.  raised  by  Gravity ;  and  forced  down  by  At- 
mospheric Pressure. 

LESSON  xxx. —  „ 

9.  Low    Pressure-engines    generally    work    with 

steam  of  about  1£  atmospheres.  The  steam- 
pressure  in  High  Pressure-engines  is  often  as 
high  as  15  atmospheres. 

10.  In  the  locomotive,  steam  causes  (by  means  of 

a  sliding-valve)  the  back-and-forth  motion  of 
the  piston  in  the  cylinder ;  and  by  this  mo- 
tion, the  back-and-forth  motion  of  the  piston- 
rod  ;  and  by  this,  the  revolution  of  the  large 
wheels.    The  wheels  roll  on  the  track ;  this  causes 
the  locomotive  to  move  onward  and  draw  the  cars 
attached  to  it. 


11.  On  dropping  a  stone  to  the  floor,  the  floor  and 

the  air  over  the  floor,  commence  vibrating. 
This  shows  that  Force  (Force  of  Gravity  in 
this  case)  may  be  converted  into  Motion. 

12.  The  motion  of  a  train  of  cars  heats  the  axles 

and  wheels  of  the  cars.  This  shows  that 
Motion  is  convertible  into  Heat. 


120  FIRST   LESSONS   IN   PHYSIOS. 

13.  Heat  expands  all  bodies  (Less.  XXVII);  and 

as  expansion  (the  work  done  by  heat)  is  mo- 
tion, we  may  say  that  Heat  is  also  converti- 
ble into  Motion.  (Thermometer.) 

14.  Heat  expands  water  into  steam.     Steam  ex- 

pands still  farther.  The  particles  of  steam, 
therefore,  are  in  continual  motion.  The  ef- 
fect of  this  motion  is  the  Expansive  Force  of 
Steam.  This  shows  that  Motion  is  convertible 
into  Force.  (Compare  Less  XXII,  Review.) 

15.  The  Expansive  Force  disappears  as  soon  as 

the  steam  has  moved  the  piston  of  the  engine. 
The  motion  of  the  piston  is  the  work  done  by 
the  steam.  Thus,  in  this  case,  Force  is  con- 
verted into  Motion.  (Compare  No.  14,  above, 
and  Less.  XXII,  No.  13.) 

16.  Force  of  Pressure  is  convertible  into  Motion 

of  Masses.     (Wind— Barometer— Pumps.) 
17  From  all  the  preceding,  we  see  that 

a.  Force  is    convertible    into    Motion.      (The 

•pump.) 

b.  Motion  is  convertible  into  Heat.     (Friction.) 

c.  Heat  is  convertible  into  Motion.    (Thermom- 

eter.) 

d.  Motion  is  convertible  into  Force.    (Expan- 

sive Force  of  Steam.) 


LIGHT — ITS  SOURCES — DIRECTION.  121 


LESSON    XXXII. 

LIGHT — ITS  SOURCES — DIRECTION. 

Familiar  Facts.  —  In  the  daytime,  whether 

the  sun  is  visible  or  not,  we  can  see  objects 
around  us.  But  we  can  not  see  objects  at 
night,  for  then  it  is  dark ;  the  sun  is  on'  the  other 
side  of  the  earth.  The  light  of  the  stars,  or  flashes 
of  lightning,  may  somewhat  relieve  the  darkness 
of  night;  glow-worms  may  feebly  illuminate  our 
immediate  vicinity.  If  we  rub  a  match  in  the 
dark  against  the  hand,  the  phosphorus  will  shine 
on  the  hand.  This  property  is  called  Phosphor- 
escence. Glimmers  of  light  are  also  noticeable  in 
decaying  animal  and  vegetable  substances.  Two 
pieces  of  sugar,  after  being  rubbed  together,  also 
emit  light.  Candles,  oil  and  gas,  at  times  also 
torch-lights,  are  our  usual  means  of  illumination. 
But  our  greatest  luminary  is  the  sun. 

1.  The  Sun  and  the  Fixed  Stars,  Electricity^ 
Phosphorescence  and  Burning  Substances  are 
Sources  of  Light.  The  sun,  stars,  lightning, 
phosphorus,  glow-worm  and  flame  are  Self-lu- 
minous Bodies. 

Familiar  Facts. — The  moon  sends  light  to  us ;, 
so  do  other  planets.  But  this  light  is  not  her  own  j 


122  FIRST  LESSONS   IN  PHYSICS. 

she  receives  it  from  the  sun,  the  same  as  the  other 
planets  do.  She  is  invisible  when  her  non-illu- 
mined portion  faces  us.  When  a  room  is  dark  a 
Ibook  upon  the  table  can  not  be  seen ;  neither  can 
the  table,  nor  the  desks,  nor  the  streets,  nor  any 
thing  else.  None  of  these  objects  is  self -lumi- 
nous;  that  is,  in  order  to  be  seen,  these  objects 
need  light  from  some  luminous  body. 

2.  Neither  the  planets,  nor  most  of  the  objects 
surrounding  us,  are  self-luminous  bodies. 

Familiar  Facts. — If  we  close  our  eyes  we  can 

not  see.  Nor  can  persons  who  were  born  blind, 
or  have  become  blind  from  accident  or  disease. 
In  order  to  see  objects  behind  us,  we  must  turn 
around ;  to  see  things  above  us,  we  must  turn  our 
eyes  upward. 

3.  Bodies  not  self-luminous  are  visible  only 
when  they  receive  light  from    some    luminous 
body;  and  then  only,  if  a  part  of  that  light 
forms  an  impression  on  our  eye. 

Pencils,  crayons,  glass,  water,  ice,  trees,  houses,  and  all  other  ob- 
jects are  seen  by  us,  because  when  light  falls  upon  objects,  a  portion  of 
the  light  is  diffused  from  their  surface  in  all  directions,  and  because  a 
small  portion  of  that  diffused  light  enters  our  eye  and  forms  an  im- 
pression on  the  retina.  From  our  room  we  see  objects  out-doors  very 
clearly;  but  when  looking  from  without,  objects  in  the  room  are  not 
seen  so  well.  The  amount  of  light  diffused  in  a  room  is  much  smaller 
than  that  diffused  out-doors. — It  is  light  in  daytime,  although  it  may  be 
very  cloudy.  The  clouds  receive  all  the  light  from  the  sun,  and  diffuse 
a  portion  of  it. 


LIGHT — ITS  SOUEOES — DIRECTION.  123 

61.  EXPERIMENT.— Place  a  large  paste-board 

(with  a  small  hole  in  it)  a  few  inches  from  the 
blackboard.  Light  a  candle  and  place  it  in  front 
of  the  hole  in  the  pasteboard.  A  bright  spot  will 
be  seen  on  the  blackboard.  It  is  a  spot  illumined 
by  the  rays  of  the  light  that  pass  from  the"  flame 
through  the  opening.  The  direction  from  the  flame 
through  the  hole  to  the  illumined  spot  is  that  of  a 
straight  line.  Let  the  flame  be  moved  about,  the 
spot  will  move  also. 

Familiar   Facts.— Through  the  cracks  in  the 

shutter  of  a  darkened  room,  rays  of  light  are  ob- 
served to  enter  in  straight  lines.  The  hunter  levels 
his  gun  at  a  squirrel  in  the  direction  in  which  the 
rays  of  light  diffused  from  the  squirrel  enter  his 
eye.  Opera-glasses  and  telescopes  have  straight 
tubes. 

4.  Light  emanates  from  self-luminous  bodies 
in  all  directions,  and  travels  in  straight  lines. 

Bead  "  Sun,  Moon  and  Stars  "  in  "The  Wonders  of  the  Heavens" — 

Illustrated  Library  of  Wonders. 

Bead  "Light  and  Color,"  in  "The  Earth  and  its  Wonders." 
Bead  "The  Eye,"  in  "The  Human  Body  "—Illustrated  Library  of 

Wonders. 


124  FIRST   LESSONS   IN   PHYSICS. 


LESSON    XXXIII. 

RADIANT    AND    SPECULAR   REFLECTION. 

During  the  daytime,  sunlight  is  diffused  in  the 
atmosphere  as  well  as  in  the  air  of  our  rooms, 
whether  the  sun  is  visible  or  not.  Some  of  this 
light  in  the  air  falls  upon  the  walls  and  upon  the 
objects  in  the  room ;  and  the  walls,  as  well  as  the 
objects,  reflect  (throw  back)  that  light  in  all  di- 
rections. They  reflect  it  thus:  Every  point  of 
their  surface  radiates  the  light  in  all  directions ; 
hence  any  point  of  this  surface  may  be  seen  by  a 
person  in  the  room,  whatever  part  of  the  room  he 
may  be  in,  provided  that  a  portion  of  that  reflected 
light  strikes  his  eye. 

Familiar  Facts. — Here  is  a  pencil.  What  ena- 
bles us  to  see  it?  It  is  not  a  self-luminous  body  ; 
but  there  is  diffused  light  in  the  room,  and  as  the 
pencil  has  a  more  or  less  rough  surface,  every 
point  on  that  surface  receives  some  of  this  dif- 
fused light,  and  in  turn  reflects  some  of  it.  It 
does  so  by  radiating  the  liglit  in  all  directions. 
Of  this  radiated  light,  a  portion  enters  our  eye, 
and  we  say  "  we  see  the  pencil,"  and  may  then 
describe  it. 

We  see  a  looking-glass,  owing  to  the  light  which 
is  reflected  from  it  by  radiation.  True,  its  surface 


• 
LIGHT,   CONTINUED.  125 

is  smoother  than  that  of  the  pencil,  or  of  most 
objects;  yet  even  in  a  looking-glass  there  are  very 
many  uneven  places,  from  every  point  of  which 
light  is  reflected  by  radiation.  Were  it  not  for 
that,  we  would  not  see  the  glass  at  all.  The  sur- 
face of  a  perfect  mirror  would  be  invisible. 

All  bodies  reflect  light  by  radiation.  We  call 
this  Radiant  Reflection  of  Light. 

62.  EXPERIMENT.— If  an  India-rubber  ball  be 

thrown  upon  the  floor  in  the  direction  in  which  a 
ray  of  light  would  pass  through  a  crack  on  the 
floor  of  a  darkened  room,  the  ball  will  rebound, 
and  may  be  made  to  strike  the  wall  opposite  the 
crack.  Let  the  place  where  it  strikes  the  wall 
be  marked.  Now  lay  a  looking-glass  upon  the 
bright  spot  on  the  floor  of  the  darkened  room 
(the  spot  is  caused  by  the  rays  of  light  entering 
through  the  crack),  and  it  will  be  seen  that  the 
rays,  like  the  India-rubber  ball,  rebound  to  where 
the  ball  struck  the  wall,  nearly.  The  rays  are 
reflected  by  the  looking-glass.  Any  other  highly 
polished  surface  would  have  caused  the  same  re- 
flection. 

All  this  shows  that  there  are  objects  which  not 
only  reflect  light  by  Radiant  Reflection  in  all 
directions,  but  which,  in  addition,  reflect  an 
extra  amount  of  light  in  certain  definite  direc- 
tions. We  call  this  Specular  (mirror-like)  Re- 
flection of  Light. 


126  FIEST   LESSONS   IN   PHYSIOS. 

Familiar  Facts.— Burnished  metal  plates,  pol- 
ished wood,  the  surface  of  water  or  mercury,  the 
coating  of  mercury  in  looking-glasses,  and  even 
common  glass  plates  when  viewed  in  a  very  ob- 
lique position,  exert  both,  "Radiant"  and  "Specu- 
lar" reflection  of  light.  Objects  with  polished 
surface  reflect  light  radiantly  and  specularly. 


Light  reflected  Radiantly  compared  with  Light 
reflected  Specularly. 

Four  points  in  common : 

1.  Both  have  emanated  first  from  a  self-lumin- 

ous body. 

2.  Both  have  been  thrown  back  from  the  sur- 

face of  bodies  not  self-luminous. 

3.  Both  have  their  rays  travel  in  straight  lines. 

4.  Both  may  enter  the  eye. 
Three  points  of  difference : 

1.  Radiantly  reflected  light  proceeds  from  the 

surface  of  all  bodies ; 

Specularly  reflected  light  only  from  the  sur- 
face of  highly  polished  objects. 

2.  Radiantly  reflected  light  is  thrown  back  from 

a  surface  in  all  directions ; 
Specularly  reflected  light  is  thrown  back 
from  a  surface  only  in  certain  directions. 

3.  Radiantly  reflected  light  enables  us  to  see 

objects  ; 

Specularly  reflected  light  enables  us  to  see 
images  of  objects. 


LIGHT,  CONTINUED.  127 


LESSON    XXXIV. 

VISIBLE  DIRECTION.— REFRACTION. 

Familiar  Facts.— -1.  When  a  person  is  hit 
with  a  stone  he  does  not  seek  the  person  who 
threw  the  stone,  in  the  direction  in  which  the  stone 
flies,  but  in  the  direction  from  which  it  comes. 
Thus,  he  whose  forehead  has  been  struck  by  a 
stone  in  a  downward  direction,  looks  upward  for 
the  perpetrator ;  he  supposes  him  to  be  in  the  di- 
rection from  which  the  stone  came.  If  struck  by 
a  stone  in  an  upward  direction,  he  will  look  down- 
ward to  find  the  evil-doer. 

It  is  the  same  with  a  ray  of  light. 

2.  A  boy  looking  at  a  steeple,  receives  its  top- 
most ray  of  light  in  downward  direction.  Imagine 


Image  Inverted. 


this  ray  to  be  extended  after  passing  through  the 
opening  in  his  eye  (Fig.  26). 


128  FIRST   LESSONS   IN  PHYSIOS. 

Since  light  travels  in  straight  lines  (Lesson 
XXXII),  the  ray  strikes  the  lower  part  of  his  eye 
in  a  downward  direction.  The  lowest  ray  coming 
from  the  foot  of  the  steeple  would,  if  extended, 
traverse  the  upper  part  of  his  eye  in  upward  di- 
rection. But  if,  turning  round  and  then  bending 
his  head  down  to  his  knees,  he  looks  at  the 
steeple  from  between  his  knees  (Fig  27),  the 


Eye  Inverted. 


FIG.  27.  Image  Upright. 

topmost  ray  will,  if  extended,  pass  downward 
through  the  upper  part  of  his  eye ;  and  the  lowest 
ray,  upward  through  the  lower  part  of  his  eye.  In 
this  case  he  would  see  the  steeple  inverted,  were  it 
not  for  the  fact  that  the  eye  sees  an  object,  or  any 
part  of  an  object,  in  the  direction  from  which  the 
rays  of  the  object  come.  All  bodies  appear  to  be 
situated  in  the  direction  from  which  their  rays 
enter  the  eye. 

63.  EXPERIMENT. — Immerse  a  pencil  in  water 
perpendicularly;  it  looks  as  straight  as  before. 
But  when  immersed  obliquely,  it  appears  bent,  or 
broken.  Oars,  when  partly  immersed,  present 


LIGHT,  CONTINUED.  129 

the  same  appearance.  When  the  pencil  is  out  of 
the  water,  we  see  it  by  means  of  the  light  diffused 
from  it  (Lesson  XXXIII).  Consequently,  when 
the  pencil  is  partly  immersed,  we  see  the  portion 
above  the  liquid  for  the  same  reason.  The  light 
diffused  from  the  immersed  portion,  however, 
must  first  travel  through  the  water,  and  then 
through  the  air.  Now,  since  the  immersed  por- 
tion seems  to  be  bent,  it  follows  that  the  rays  dif- 
fused from  it  are  bent;  that  is,  they  travel  in 
straight  lines  through  the  liquid,  but  on  entering 
the  air,  they  are  made  to  deviate  from  their 
straight  course.  But  the  eye  is  in  the  habit  of 
following  the  direction  of  the  rays,  and  must  see 
the  pencil  bent  simply  because  the  rays  coming 
from  it  are  bent. 

Let  a  b  be  the  pencil  partly  immersed  ;  the  part 
immersed,  a  c,  appears  to  be  at  d  c,  because  the 
ray  coming  from  a,  which  ought  to 
pass  out  in  the  direction  of  a  &,  is 
made  to  deviate  from    its   course 
when  leaving  the  water  a    c,  and 
enters  the  eye  in  the  direct  on  of  d 
e.  The  eye,  believing  the  point  a  to 
FIG.  28.         be  in  the  direction  from  which  its 
ray  comes,  sees  the  point  a  actually  as  being  at  d. 
The  same  takes  place  with  the  other  rays  entering 
the  eye ;  hence  the  whole  part  a  c  is  seen  as  being 
at  do. 
9 


130  FIEST   LESSONS   IN   PHYSIOS. 

64.  EXPERIMENT. — A  coin  placed  on  the  bottom 
of  a  filled  tumbler,  is  seen  in  its 
true  direction  if  viewed  perpendicu- 
larly; but  if  viewed  obliquely,  it 
will  be  seen  in  a  more  elevated 
place. 

FIG.  29. 

Familiar  Facts.— Owing  to  refraction  of  light, 

the  bottom  of  clear  waters  appears  to  be  more 
elevated  than  it  really  is ;  that  is,  water  often  ap- 
pears less  deep  than  it  in  reality  is.  This  must  be 
taken  into  account  by  persons  bathing,  so  that 
they  may  not  go  beyond  their  depth. 

Hays  of  light,  on  passing  obliquely  through 
substances  of  different  densities  (such  as  air 
and  water,  or  glass  and  water),  demote  from 
their  straight  course;  they  are  bent.  This  de- 
viation is  called  Refraction  of  Light. 


PEISMS — LENSES. 


131 


LESSON   XXXY. 

PEISMS. — LENSES. 

65.  EXPEEIMENT.— On  a  blackboard  make  a 

mark  in  the  shape  of 
an  arrow,  and  look 
at  it  through  a  glass 
prism,  which  should 
be  held  so  that  only 
one  edge  of  it  is  di- 
rected upward  (Fig. 
30).  The  arrow  will 
then  be  seen  as  being  above  its  true  place.  The 
reason  is  this  :  Rays  of  light,  a  e — we  take  but 
two  for  the  sake  of  simplicity — diffused  from  the 
arrow,  strike  the  surface,  b  c,  obliquely,  and  are, 
therefore,  refracted  to  /  (Less.  XXXIV ).  On  pass- 
ing from  the  glass  prism  at  /,  they  are  again  re- 
fracted, and  enter  the  eye  which  is  stationed  at  g. 
But  the  eye  follows  the  direction  of  the  refracted 

rays  (Less.  XXXIY); 
consequently  it  sees 
the  arrow  as  being 
at  Ti. 

66.    EXPEEIMENT. — 

Now  look   at  the 

k\   ,/         v  arrow  on   the  black- 

FIO.SI.  board  through  the 


132 


FIRST   LESSONS   IN   PHYSICS. 


prism  inverted  (Fig.  31);  that  is,  placed  so  as  to 
present  two  edges  upward.  The  arrow  will  then 
be  seen  below  its  true  place.  The  reason  of  this 
is  the  same  as  before.  The  rays  a  e  are  refracted 
to/;  consequently  the  arrow  is  seen  as  being  at#. 
If  now  we  place  two  prisms  together,  as  in 
Fig.  32,  rays  diffused  from  the  arrow  and  entering 


PIG.  33. 


the  glass  surface,  a  I  c,  will,  in  like  manner,  be 
refracted  twice,  and  meet  each  other  in  several 


PEISMS — LENSES. 


133 


points  behind  the  prism.  In  order  that  the  eye 
of  a  person  situated  at  /,  might  receive  all  these 
refrn^ted  rays,  and  thus  be  able  to  see  the  whole 
arrow  through  the  prisms,  it  would  be  necessary 
to  have  these  rays  blend  into  a  common  point. 
To  do  this,  we  must  have  the  surface,  a  ~b  c,  curved 
(Fig.  33) ;  that  is,  we  must  have  a  curved  glass  in 
place  of  the  prisms.  Such  a  curved  glass  is  a 
convex  Lens,  commonly  called  a  Burning-glass. 
For  it  not  only  brings  rays  of  light  to  a  common 
point,  the  Focus,  but  at  the  same  time  it  blends 
rays  of  heat  into  a  focus.  In  consequence  of  this, 
a  match  ignites,  and  a  hole  is  burnt  in  a  piece  of 
paper,  if  either  of  these  objects  be  held  in  the 
focus. 


FIG  84. 


The  arrow,  as  viewed  through  a  lens,  is  seen 
larger  (Fig.  34) ;  that  is,  it  is  magnified,  because — 


134  FIKST   LESSONS   IN   PHYSIOS. 

taking  the  two  extreme  rays  for  the  sake  of  illus- 
tration— the  rays,  d  e  and  f  g,  refracted  to  the 
eye,  are  seen  as  coming  from  the  points  Ji  L 
(Why?  Lesson  XXXIY,  p,  130.)  The  common 
Burning-glass,  therefore,  is  also  called  Magnify- 
ing-Glass. 

An  object  in  front  of  a  convex  lens  is  seen  mag- 
nified ~by  an  eye  placed  behind  tlie  lens. 

Application. — The  lenses  in  spectacles  ;  opera- 
glasses  and  telescopes ;  all  magnifying  glasses. 

Read  "Magnifying  and  Burning  Glasses,"  in  "Pursuit  of  KnowK 
edge,"  Vol.  II.  Harper  &  Bros. 

Bead  "Lenses,"  in  "The  Wonders  of  Optics"— Illustrated  Library  of 
Wonders. 

Head  "How  to  View  Pictures,"  p.  248,  in — "Spectacles,"  p.  250 — m 
"Things  not  Generally  Known." 


OOLOES.  135 


LESSON    XXXVI. 

C  O  LO  E. 

66.  EXPEEIMENT.— If  a  large  pasteboard  with 
a  small  hole  be  placed  facing  the  sun  ;  or  if  a 
room  be  darkened  and  only  a  few  rays  of  light 
admitted  through  a  crack  in  the  shutter,  these 
rays  will  pass  to  the  floor  and  there  form  a  spot 
of  white  light.  But  if  a  prism1  be  held  before 
the  crack  or  before  the  hole  in  the  pasteboard, 
the  rays  of  light  will  be  refracted  (Less.  XXXIV), 
and  spread  out  in  the  form  of  a  long  band.  In- 
stead of  being  white,  this  band  will  be  colored. 
The  colors  of  the  rainbow  may  be  distinguished 
in  it ;  viz. :  Violet,  indigo,  blue,  green,  yellow, 
orange  and  red.  The  colored  band  is  called  the 
Solar  Spectrum ;  and  this  spreading  out  of  light 

I.  A  prism  which  is  to  show  the  refraction  of  light,  may  be  of  solid 
glass,  or,  if  such  a  one  can  not  be  had,  it  may  be  constructed  in  the  fol- 
lowing manner :     Procure  two  strips  of  common 
glass,  having  the  shape  of  a  rectangle,  each  of  the 
same  size,  about  5  inches  long  by  i^  inches  wide. 
FIG.  35  One  of  the  long  edges  of  each  is  heated  over  an 

alcohol  flame;  both  edges  are  then  cemented  together  with  sealing-wax, 
allowing  a  distance  of  l%  inches  between  the  two  remaining  long  edges. 
The  ends  of  the  vessel  thus  formed  are  closed  by  triangular  pieces  of 
thin  board,  measuring  \yz  inches  on  each  side,  and  which  are  likewise 
cemented  to  the  glass.  Water  is  then  put  in,  and  when  used,  the  prism 
is  held  so  as  to  have  the  long  cemented  edge  below. 


136  FIRST   LESSONS  IN   PHYSICS. 

is  called  Dispersion.  If  the  spectrum  be  made  to 
fall  upon  a  mirror,  it  will  be  reflected  in  straight 
lines  like  ordinary  light. 

67.  EXPERIMENT.— To  convince  ourselves  that 
ordinary  sunlight  contains   the  seven  colors  of 
the  rainbow,  let  the  spectrum  produced  by  one 

prism  fall  upon  a  second 
prism  of  the  same  size 
as  the  first,  but  placed 
PIQ>86-  as  shown  in  the  figure 

annexed.  The  rays  of  light,  dispersed  by  the  first 
prism,  will  be  collected  by  the  second,  and  will 
then  produce  white  light  again. 

68.  EXPERIMENT. — The  same  may  be  shown  if 
a  top,  painted  with  the  seven  colors  of  the  rain- 
bow, is  set  spinning  rapidly.    The  impressions 
made  in  the  eye  by  these  different  colors  are 
mixed  together,  and  thus  produce  a  mixture  of 
the  colors  which  is  nearly  white. 

All  this  shows  that  white  sunlight  is  composed 
of  the  seven  colors  of  the  rainbow. 

69.  EXPERIMENT.— Between  the  crack,  or  the 
hole  in  the  pasteboard,  and  the  prism  insert  a 
piece  of  red  glass.     The  spectrum  will  then  be 
almost  entirely  red,  and  the  other  colors  be  found 
wanting.    Insert  a  piece  of  green  glass  in  place  of 
the  red ;  the  spectrum  will  be  almost  exclusively 
green ;  with  blue  glass  it  will  be  nearly  blue,  &c. 
It  is  manifest,  that  white  light  falls  upon  each 


COLOES.  137 

piece  of  colored  glass  ;  and  that  only  one  color  at 
a  time  falls  upon  the  prism.  Thus  when  the  red 
glass  is  inserted,  only  red  light  falls  upon  the 
prism,  and  consequently  there  can  be  but  a  spec- 
trum of  red  light.  The  question  now  is,  what 
becomes  of  the  remaining  light — or  colors — which 
fall  on  the  red  glass  ?  Evidently  the  red  glass 
absorbs  all  the  light  except  the  red,  and  this  it 
throws  out.  The  same  takes  place  with  each  of 
the  other  colored  glasses  ;  the  green  absorbs  all 
the  light  it  receives  except  the  green  light ;  this  it 
throws  out.  The  blue  absorbs  all  the  light  it  re- 
ceives save  the  blue,  which  it  throws  out,  &c. 
White  glass,  however,  transmits  nearly  all  the 
light,  and  absorbs  very  little  or  scarcely  any. 

70.  EXPERIMENT. — If  a  sheet  of  red  colored 
paper  be  held  facing  the  sun,  and  a  sheet  of 
white  paper  before  it,  so  as  to  form  an  oblique 
angle  with  it,  the  portion  of  the  white  paper 
which  is  near  the  red,  will  appear  red.  In  this 
case  red  rays  are  diffused  from  the  red  body  and 
fall  upon  the  white.  But  the  red  paper  receives 
white  light  from  the  sun,  hence  it  must  have  ab- 
sorbed all  of  the  white  light  save  the  red ;  this  it 
throws  out. 

Familiar  Facts.— Objects  near  a  blue  curtain 
often  have  a  bluish  hue.  The  curtain  receives 
white  light,  and  absorbs  it  all  except  the  blue, 
which  it  reflects.  Objects  near  the  foliage  of 


138  FIRST   LESSONS   IN   PHYSICS. 

trees  and  bushes  often  have  a  greenish  hue,  be- 
cause green  leaves  absorb  all  the  light  that  falls 
upon  them  save  the  green  ;  this  they  diffuse  in  all 
directions,  and  thus  send  green  light  to  the  ob- 
jects near  by. 

A  body  is  colored  when  it  reflects  only  a  por- 
tion of  wMte  light.  A  body  is  white  when  it  re- 
flects all  the  white  light;  and  a  body  is  black 
when  it  reflects  (almost)  no  light,  that  is,  when  it 
absorbs  all  the  light. 


Questions. — What  causes  a  piece  of  red  cloth 
to  appear  red  ?  It  sends  only  red  rays  to  the  eye, 
the  other  rays  it  absorbs.  What  causes  a  sheet 
of  white  paper  to  appear  white  ?  It  absorbs  no 
light,  but  rejects  nearly  all  of  it.  Some  of  this 
rejected  or  reflected  light  enters  the  eye  and  thus 
produces  the  sensation  of  white  in  us. 

What  causes  a  black  coat  to  be  black  ?  It  ab- 
sorbs nearly  all  the  light,  consequently  it  sends 
scarcely  any  to  the  eye.  The  eye  receives  just 
enough  light  from  it  to  become  aware  of  its  pres- 
ence,  but  not  sufficient  to  perceive  any  color. 

Why  is  every  thing  black  in  a  dark  night? 
Because,  when  there  is  no  light,  objects  receive 
none,  and,  therefore,  they  can  not  send  any  to  the 
eye.  But  if  no  light  enters  the  eye,  we  see 
nothing  (Lesson  XXXIII). 

Color  is  not  a  quality  inherent  in  bodies. 


COLORS.  139 

Application. — The  application  of  colors  is  so 
manifold,  that  it  is  impossible  to  mention  each. 
They  serve  to  enliven  the  scenery  around  us ,  to 
improve  our  own  appearance ,  to  indicate  joy  or 
mourning.  We  imitate  the  thousand  delicate 
hues  and  tinges  of  the  colors  in  nature  in  our 
paintings,  artificial  flowers,  and  in  many  different 
contrivances.  Colors  also  serve  as  signals  to  be 
seen  from  afar,  hence  their  use  in  lighthouses, 
on  railroads  (colored  lights),  and  with  the  mili- 
tary (flags),  &c.,  &c. 

Bead  "  Color- Blindness,"  p.  242,  and  "Principles  of  Harmony  and 
Contrasts  in  Color,"  p.  244,  in  "Things  Not  Generally  Known." 

Bead  "  Color,"  in  "The  Earth  and  its  Wonders." 


140  FIRST   LESSONS   IN   PHYSICS. 


LESSON    XXXVII. 

CHEMICAL   ELECTRICITY. 

71.  EXPERIMENT. — Take  a  plain  glass  tumbler, 

and  place  in  it  a  porous  cup  of  earthenware  (un- 
glazed)  in  a  manner  such,  that  between  the  cup 
and  the  tumbler  there  is  a  finger's  width  of  space 
left.  Next  have  a  small  sheet  of  zinc  cut  as  high 
as  the  cup.  Then  bend  it  into  a  cylinder  wide 
enough  to  encircle  the  porous  cup  freely.  This 
cylinder  is  open  above  and  below,  with  a  slit 
through  its  whole  height.  On  the  top,  and  oppo- 
site the  slit,  about  a  square  inch  of  zinc  is  left 
higher  than  the  rest.  To  this  piece,  one  end  of  a 
copper  wire  about  a  foot  long  is  soldered.  The 
zinc  cylinder  is  put  into  the  space  between  the 
tumbler  and  the  cup ;  the  space  is  then  filled  with 
diluted  sulphuric  acid  (a  table-spoon  full  of  the 
acid  mixed  with  ten  times  the  quantity  of  water). 
The  cup  is  filled  with  strong  nitric  acid.  In  the 
acid  place  a  plate  of  carbon,  to  the  top  of  which 
the  end  of  another  copper  wire  is  secured.  If  no 
carbon  plate  can  be  had,  a  narrow  strip  of  pla- 
tinum may  be  used,  and  another  wire  soldered  on 
it.  If  that,  too,  can  not  be  obtained,  fill  the  cup 
with  crushed  coke.1  Thus  prepared,  the  cup  is 

I.  The  filling  with  coke  must  be  done  in  the  following  manner :  Coke 
is  pulverized  in  a  mortar,  then  a  small  quantity  is  first  put  in  the  cup 


CHEMICAL   ELECTEICITY.  141 

placed  inside  the  zinc  cylinder ;  the  diluted  acid, 
of  course,  surrounding  it.  Such  an  apparatus  is 
called  a  cell  or  element ;  if  two  or  more  cells  are 
connected  with  each  other,  the  apparatus  is  called 
a  battery. 

The  free  end  of  the  wires  must  be  scraped  clean 
with  a  file  or  knife.  If,  then,  they  are  brought 
quite  near  to  each  other,  a  small,  bright  spark 
is  produced.  If  the  tongue  is  held  between  the 
two  ends,  a  thrilling  sensation  is  felt. 

In  the  first  place,  the  diluted  acid  acts  upon 
the  zinc;  this  action  may  be  seen  by  the  minute 
bubbles  rising  from  the  zinc;  it  may  also  be 
heard.  They  are  bubbles  of  a  gas  called  hydro- 
gen. In  the  second  place,  we  have  carbon,  or 
platinum,  in  contact  with  nitric  acid;  the  action 
which  takes  place  here  is  invisible.  Thirdly,  the 
two  liquids  penetrate  the  porous  cup,  and,  there- 
fore, meet  with  each  oilier.  This  action  is  in- 
visible also. 

The  mutual  contact  of  two  different  metals 
(or  of  zinc  and  carbon),  each  placed  in  a  certain 
liquid,  produces  Chemical  Electricity. 

This  electricity  is  also  called  "  Galvanic  Elec- 

and  a  little  nitric  acid  mixed  with  it,  so  that  the  powder  may  be  soaked 
with  acid.  This  is  repeated  several  times,  until  the  cup  is  nearly  filled 
with  saturated  coke.  On  top  of  the  coke  a  lump  of  coke  is  placed, 
around  which  a  copper  wire  is  wound  several  times,  so  that  about  a  foot 
length  of  wire  remains  free.  That  the  coke  lump  may  stand  firmly, 
surround  the  lower  part  of  it  by  coke  powder. 


142  FIRST   LESSONS   IN   PHYSICS. 

tricity,"  because  it  was  discovered  by  Galvani, 
an  Italian  physician,  toward  the  end  of  the  last 
century. 

The  electric  spark  is  seen  only  when  the  free 
ends  of  the  wires  are  brought  together.  The  zinc 
is  in  contact  with  the  diluted  acid;  electricity 
passes  from  the  zinc  to  the  acids,  and  thence  to 
the  carbon,  and  from  the  carbon,  electricity,  that 
is  the  invisible  electric  current,  passes  along  the 
copper  wire,  returns  to  the  zinc,  then  to  the  diluted 
acid  again,  and  so  forth,  in  the  same  manner  as 
above,  forming  an  uninterrupted  current  of  Elec- 
tricity. If  the  metals  (wires)  are  not  very  near 
to  each  other,  no  spark  is  seen,  and  the  current  is 
interrupted.  If  the  end  of  one  of  the  wires  be 
attached  to  a  pair  of  scissors,  the  spark  will  be 
seen  at  the  point  of  the  scissors  on  bringing  it 
very  near  to  the  other  wire. 

Chemical  Electricity  is  also  produced  by  two  different  metals  im- 
mersed in  only  one  liquid.  Thus,  a  simple  battery  can  be  made  by 
taking  a  piece  of  sheet-zinc,  and  one  of  sheet-lead,  e  ach  about  one-fourth 
the  size  of  this  page,  and  separated  from  each  other  by  a  cloth  or  blot- 
ting paper  a  little  larger  than  the  metals.  A  few  feet  of  thin  copper- 
wire  conveniently  fastened  to  each  metal  near  the  edge,  and  the  lead 
may  be  placed  in  a  dish,  then  the  cloth  or  pnper,  and  the  zinc  on  top. 
The  whole  is  now  covered  with  a  solution  of  blue  vitriol  in  water. 
Another  simple  battery  can  be  fitted  up  from  a  strip  of  zinc  and  a  piece 
of  carbon,  immersed  in  a  solution  of  bi-chromate  of  potash,  mixed  with 
a  small  quantity  of  sulphuric  acid.  Either  cell  will  suffice  to  work  an 
ordinary  electro-magnet. 


THE  ELECTRO-MAGNETIC   TELEGRAPH.         143 

LESSON    XXXVIII.    * 

THE   ELECTRO-MAGNETIC   TELEGRAPH. 

Next  to  the  steam-engine  the  telegraph  forms 
the  wonder  of  our  age.  Its  eminent  usefulness 
and,  more  yet,  the  incredible  rapidity  with  which 
it  communicates  messages  from  one  place  to  an 
other,  is  something  so  new,  so  extraordinary,  that 
we  are  tempted  to  believe  there  is  nothing  which 
the  human  mind  is  not  capable  of  achieving. 

The  fire- signals  of  the  ancients  were  no  longer 
sufficient  for  the  increasing  demands  of  civiliza- 
tion. Toward  the  end  of  the  last  century,  so-called 
"  optical"  telegraphs,  consisting  of  high  poles 
erected  upon  high  buildings  or  hills  were  used  in 
France.  By  means  of  moveable  arms  attached  to 
them,  signs  could  be  made  which  in  clear  weather 
were  visible  at  great  distances.  But  when,  in  1820, 
it  had  been  discovered  by  Oerstedt,  a  Danish  pro- 
fessor, that  the  electric  current  running  along  a 
wire,  exerted  a  certain  influence  upon  iron,  it  was 
at  once  proposed  to  apply  that  influence  to  the 
telegraph. 

The  first  electric  wire  by  which  messages  were 
sent,  was  put  up  by  Steinheil,  between  his  place 
of  residence  in  Munich  and  the  astronomical 
observatory  near  that  city.  England  soon  fol- 


144  FIRST   LESSONS   IN   PHYSICS. 

lowed  the  example;  so  did  America.  As  is 
always  the  case  with  new  inventions,  a  great 
many  improvements  were  made  in  rapid  succes- 
sion. It  was  an  American,  Morse,  who,  by  a  very 
simple  but  ingenious  improvement,  brought  the 
telegraph  to  its  present  degree  of  perfection. 

The  principle  of  Morse's    telegraph  may  be 
illustrated  easily  by  the  following  experiment : 

72.  EXPERIMENT. — A  cylindrical  rod  of  soft 
iron  is  bent  into  the  shape  of  a  horse-shoe.  The 
rod  may  be  J  inch  in  diameter  and  10  inches  long. 
Its  two  ends  must  be  filed  smooth  ;  the  whole  is 
then  covered  with  clay  and  placed  in  a  coal  fire. 
There  it  is  left  for  a  time  and  allowed  to  coo] 
gradually,  when  the  fire  has  gone  out.  After  this 
the  clay  is  removed,  and  the  two  ends  filed  smooth 
again.  Then  take  a  coil  of  copper  wire  of  about 
of  an  inch  in  diameter,  heat  it  red-hot  and  cool 
it  in  water.  The  wire  must  then  be 
completely  insulated,  by  either  wrap- 
ping it  in  silk  or  in  paper  ;  or,  as 
the  insulation  may  be  very  thin,  by 
varnishing  it.  The  trouble  of  heat- 
ing and  insulating  is  avoided  by 
buying  insulated  wire.  It  is  then 
FIG.  ST.  wrapped  round  the  iron  in  close 
windings.  (See  Fig.  37.)  When  beginning  to  wrap, 
leave  about  two  feet  of  wire  free,  wind  then  closely 
near  to  the  bend ;  leave  the  bend  uncovered,  and 


ELECTRO -MAGNETIC   TELEGRAPH.  145 

stretch  the  wire  across  to  the  other  arm.  Then 
proceed  downward  to  the  other  end,  and  leave  the 
last  two  feet  of  the  wire  again  free.  Both  ends  of 
the  wire  are  to  be  scraped  clean,  and  afterward 
connected  with  the  wires  of  the  galvanic  element, 
so  that  the  wire  starting  from  the  carbon  (or 
platinum)  be  connected  with  one  of  the  wires  of 
the  horse- shoe  ;  and  the  wire  of  the  zinc  cylinder, 
with  the  other  wire  of  the  same.1 

If  now  a  piece  of  soft  iron,  smooth  on  one 
side,  or  a  nail,  be  held  at  a  short  distance  from 
the  ends  of  the  bent  rod,  it  will  be  attracted  by 
them,  and  adhere.  Tlie  electricity  flowing  around 
the  iron  rod,  has  rendered  the  rod  magnetic  ;  its 
ends  are  now  magnetic  poles.  (See  Lesson  III.) 

The  galvanic  current  now  travels  from  the  car- 
bon along  the  wire,  passes  through  the  place 
where  the  wires  are  fastened  together,  and  enters 
the  wire  leading  to  the  horse-shoe.  Then  it  runs 
through  all  the  windings  of  the  two  coils,  and,  in 
doing  so,  constantly  flows  around  the  iron  rod. 
Leaving  the  iron  rod  at  the  other  end,  it  passes 
along  the  copper  wire,  enters  the  (zinc)  wire  where 
the  two  wires  are  connected  with  each  other,  and, 
finally,  arrives  again  at  the  zinc,  whence  it  starts 
again  to  make  the  same  travel  anew. 

i .  The  connection  may  be  effected  either  by  holding  the  two  respective 
wire-ends  firmly  together  with  both  hands,  or  by  twisting  them  closely 
together. 

10 


146  FIRST   LESSONS   IN   PHYSICS. 

Disconnect  one  of  the  wires,  either  by  withdraw- 
ing one  hand,  or  by  untwisting  the  wire  ends ;  if 
the  iron  rod  is  of  the  right  kind,  the  piece  of  soft 
iron  attached  will  drop  instantly.  If  held  up 
against  the  poles  again,  it  will  not  be  attracted 
so  long  as  the  wires  remain  disconnected.  The 
iron  rod  shows  no  trace  of  magnetism.  Evidently 
it  was  magnetic  only  as  long  as  the  electric  cur- 
rent flowed  around  it.1 

Iron  becomes  magnetic  when  an  electric  current 
passes  around  it ;  but  at  the  instant  that  the  cur- 
rent is  interrupted  it  ceases  to  be  magnetic. 

Such  an  iron  rod  may  have  the  shape  of  a  horse- 
shoe, or  of  two -spools.  It  is  an  Electro-Magnet. 
The  piece  of  soft  iron  applied  to  its  poles  is  called 
the  Keeper. 


Principles  of  the  Electric  Telegraph. 

I.  According  to  Lesson  III,  magnets  have  the 
power  of  attracting  iron  ;  by  means  of  alternately 
closing  and   breaking   the    electric    current,  the 
electro-magnet  renders  a  piece  of  soft  iron  alter- 
nately magnetic  and  unmagnetic. 

II.  The  length  of  the  wires  connecting  the  gal- 
vanic battery  with  the  electro-magnet   is  imma- 

I  In  most  cases  some  electricity  is  left  after  the  current  has  been  in- 
terrupted.    It  lasts,  however,  but  a  short  time. 


ELECTRO-MAGNETIC   TELEGRAPH. 


147 


terial ;  it  may  be  thousands  of  miles.  Thus  a 
battery  may  be  in  the  city  of  New  York,  while 
the  electro-magnet  with  which  it  is  connected  is 
set  np  in  St,  Louis,  a  distance  of  1200  miles,  nearly. 
III.  A  person  stationed  at  the  battery,  may, 
by  disconnecting  and  connecting  the  wires,  break 
and  close  the  current  at  Ms  pleasure. 


The  three  principles  can  be  demonstrated 
by  a  simple  apparatus  shown  in  Fig.  38.  Two  up- 
right pieces  of  board,  M  N^  are  fastened  to  a  table 
so  as  to  admit  the 
wooden  piece,  #, 
between  them.  The 
horse- shoe  rod,  J., 
is  made  an  electro- 
magnet  whenever 
the  wires  are  pro- 
p  e  r  1  y  connected 
with  the  galvanic 
element.  A  piece 
of  soft  iron,  d  e,  on 
which  thin  paper 
has  been  pasted,  is 
attached  to  a  one 
armed  lever,  b  c> 
whose  fulcrum  is  at  D.  When  the  electric  cur- 


148  FIRST   LESSONS  IN   PHYSICS. 

rent  passes  through  A,  the  poles  of  the  Electro- 
magnet attract  the  Keeper  d  e ;  but  on  breaking 
the  current  by  disconnecting  one  of  the  wires,  the 
Keeper  will  drop.  To  prevent  its  dropping  too 
far,  there  is  a  wooden  support,  ^,  which  does  not 
allow  the  Keeper  to  separate  from  the  poles  of  the 
Electro -magnet  more  than  perhaps  1-10  of  an  inch. 
A  piece  of  wire  previously  wound  around  a  lead- 
pencil,  serves  to  draw  the  lever  promptly  down- 
ward. The  paper  pasted  on  the  Keeper  immedi- 
ately disconnects  the  latter  from  the  poles  of  the 
magnet  when  the  current  is  broken.  Lastly,  a 
wooden  point,  /,  writes  the  message  upon  an  end- 
less band  of  paper,  which  is  unwound  from  a 
cylinder  above  it.  This  cylinder  is  not  repre- 
sented in  the  drawing. 

When  the  keeper  is  attracted  by  the  magnet, 
the  point  f  makes  a  mark  or  indentation,  on  the 
paper.  But  when  the  current  is  interrupted,  the 
Keeper  drops,  and  the  point  drops  at  the  same 
time ;  consequently  no  mark  is  then  made.  To 

represent  the  letter  a,  for  example,  a  sign : , 

is  impressed  upon  the  paper ;  the  operator  at  the 
delivery  station  closes  the  current  for  an  instant 
only,  this  produces  the  small  line  — ;  then  he 
breaks  it,  but  immediately  afterward  closes  it 
again,  and  keeps  it  closed  three  times  as  long  as 

before.    This  produces  the  other  line,  ,  and 

now  the  letter  a  is  on  the  paper  of  the  operator 


ELECTRO-MAGNETIO  TELEGRAPH.  149 

In  the  receiving  station.    To  write  the  word  table, 
the  following  signs  are  necessary : 

t  a  l>  I         e 

Experienced  operators  are  able  to  write  down 
the  messages  merely  from  the  clicking  of  the  lever. 


Magnet  and  Electro-Magnet  Compared. 

Five  points  in  common  : 

1.  Both  attract  iron. 

2.  Each  has  usually  the  form  of  a  horse- shoe. 

3.  Each  has  two  poles. 

4.  In  both  the  power  resides  chiefly  at  the  ends. 

5.  Both  are  eminently  useful  to  man :  the  mag- 

netic-needle as  a  guide  upon  the  ocean ; 
the  'electro-magnet  as  a  carrier  of  messages. 
Two  points  of  difference  : 

1.  A  magnet  has  no  wire  coil  (helix)  around  it; 

an  electro-magnet  has. 

2.  A  magnet  always  attracts  iron  ; 

an  electro-magnet,  only  when  an  electric 
current  passes  around  it. 

Read  "  The  Old  Telegraphs,1'  p.  69—"  The  Laying  of  the  Atlantic 
Cable,"  p.  193,  in  "Inventions  and  Discoveries,"  by  Temple.  Groom- 
bridge.  London. 


150  FIRST  LESSONS   IN   PHYSICS. 

LESSON     XXXIX. 

EE VI E W . 

LESSON  xxxn. — 

1.  The  Sun,  the  Fixed  Stars,   Electricity,  Phos- 

phorescence, Luminous  Animals,  and  Burning 
Substances,  are  Sources  of  Light. 

2.  Neither  the  plants  nor  most  of  the   objects 

around  us,  are  self-luminous  bodies. 

3.  Bodies  not  self-luminous  are  visible  only  when 

they  receive  light  from  some  luminous  body, 
and  when  a  portion  of  that  light  forms  an  im- 
pression upon  our  eye. 

4.  Light  emanates  from  a  self-luminous  body  in 

all  directions,  and  travels  in  straight'  lines. 
LESSON  xxxin. — 

5.  All  bodies  reflect  light  radiantly. 

6.  Objects  with  polished  surface  reflect  light,  both, 

radiantly  and  specularly. 

7.  All  bodies  appear  to  be  in  the  direction  whence 

their  rays  enter  the  eye. 

8.  Rays  of  light,  on  passing  obliquely   through 

substances  of  different  density,  such  as  glass, 
water,   or  air,   deviate  from    their    straight 
.    course ;   they  are  refracted. 
LESSON  xxxv. — 

9.  An  object  before  a  convex  lens,  appears  mag- 

nified to  the  eye  situated  behind  the  lens. 


REVIEW.  151 

LESSON    XXXVI. — 

10.  White  sunlight  is  composed  of  the  colors  of 
the  rainbow. 

11.  A  body  is  colored  when  it  diffuses  only  a  por- 
tion of  the  white  light  it  receives  ;  a  body  is 
white  when  it  diffuses  all  the  white  light  it 
receives  ;  a  body  is  black  when  it  absorbs  all 
the  white  light  it  receives. 

12.  Color  is  not  a  quality  inherent  in  bodies. 

LESSON    XXXVII. — 

13.  The  mutual  contact  of  two  different  metals 

(or  of  zinc   and  carbon),   each  placed  in   a 
certain  liquid,  produces  chemical  electricity. 

14.  Chemical  electricity  travels  in  a  circuit  from 
its  source  and  back  again. 

LESSON  xxxvni. — 

15.  Soft  iron  is  magnetic,  when  an  electric  current 
passes  around  it. 

When  the  current  is  interrupted,  it  ceases  to 
be  magnetic. 

16.  The  principles  of  the  Electric  Telegraph  are : 

1.  A  piece  of  soft  iron  may  be  rendered  alter- 

nately magnetic  and  unmagnetic  by  means 
of  an  electro-magnet. 

2.  The  electric  current  travels  over  any  length 

of  wire. 

3.  A  person  stationed  at  the  electric  battery, 

may  close  and  break  the  current  at  his 
pleasure. 


QUESTIONS. 


(Questions  preceded  by  a  =  are  of  a  more  difficult  character.) 
LESSON  1.— GRAVITY. 


PAGE  1 1. — 

I.  Why  does  a  stone  in  our  hand 
not  fall  ? 

2  Why  does  it  fall  when  drop'd  ? 

3  Why  does  a  pencil  roll  down 

from  the  desk  ? 

4  Whither  does  a  stone  thrown 

into  a  pond  fall  ?  and  why  ? 

5  Whither    does   a    sign-board 

blown  off  by  the  storm  ?  and 
why  ? 

6  Whence  does  rain,  snow  and 

hailstones  come  ?  and  why  ? 

7  When  does  water  form  water- 

falls ? 

8.  Why  do  coals  fall  through  the 

grate  ? 

9.  Why  does  soot,  through   the 

air? 

10.  To  what  purpose  are  heavy 

rods  attached  to  maps  and 
curtains  ? 

11.  To  what  purpose  are  clocks 

provided  with  weights  ? 
'  12.  Why  is  it  that  all  bodies  near 
the  earth  have  a  tendency  to 
approach  the  earth? 

13.  Give  the  law  of  gravity. 

PAGE  12. — 

14.  Why  is  a  string,  with  a  weight 

attached,  drawn  straight? 

15.  What   prevent^    the    weight 

from  falling  ?    •" 

16.  What  does  the  string  indicate? 

17.  Define  vertical. 

1 8.  What  is  a  plumb-line  ? 

19.  Give  the  law  of  Direction  of 

Force  of  Gravity. 

20.  Why  does  a  large  stone  press 

itself  partly  into  the  ground? 


PAGE  12. — 

21.  Why  do  heavy  wagons  make 

ruts? 

22.  In   what    manner    do  ladies 

judge  of  silk  robes  ? 

23.  Define  weight. 

24.  What  is  a  balance  ? 

25.  What  are  the  weights  ? 
PAGE  13. — 

26.  How  does  it  come  that  a  pound 

of  coffee  has  as  much  weight 
as  a  pound  of  lead  ? 

27. a  pound  of  feathers  as 

much  as  a  pound  of  iron  ? 

28.  Of  what  force  of  Nature  is  the 

Balance  an  application  of? 

29.  Clock  weights  ? 

30.  Hour-glasses? 


31.  Why  is  a  large  drop  of  mer- 

cury lying  upon  the  table 
never  entirely  round  ? 

32.  Why   do   wagons,    unless- 

checked,  roll  down  hill  with 
great  rapidity  ? 

33.  Why  do  light  bodies,  such  as- 

feathers,  bits  of  paper,  &c., 
fall  to  the  ground  more  slow- 
ly than  heavy  bodies,  such  as 
stones  and  the  like  ? 

34.  Where  must  a  rod  be  support- 

ed to  be  evenly  balanced  ? 

35.  How  can  the  weight  of  a  body 

be  found  by  means  of  a  bal- 
anced rod  ? 

36.  Has  a  body  the  same  weight 

on  different  heavenly  bodies? 

37.  Wha^   will    a   pound  of   tea 

weigh  on  the  moon  ? 

38.  What,  on  the  sun  ? 

39.  What,  in   the  center  of  the 

earth  ? 

40.  What,  half  way  between  cen- 

ter and  surface  ? 


154 


FIRST   LESSONS  IN   PHYSICS. 


41.  Would  it  weigh  more,  or  less, 
if  at  a  considerable  distance 
above  the  surface  ? 


42.  What  causes  the  tide-waves  ? 

43.  What,  the  revolution  of  the 

moon  around  the  earth  ? 


LESSON  II.— SPECIFIC  GRAVITY. 


FLOATING  AND  SINKING  SOLIDS. 


PAGE  14. — 

44.  What  is  meant  by  the  state- 

ment "Water  is  heavier  than 
oil  ?" 

45.  How  should  the  statement  be? 

46.  Prove  that  a  pound  of  water 

is  as  heavy  as  (better:  has 
the  same  weight  as)  a  pound 
of  oil.  » 

47.  Why  does  a  pint  of    nercury 

weigh  more  than  a  pint  of 

water  ? 
.48.  Why  has   a  solid  rubber-ball 

more  weight  than  a  hollow 

one  ? 
49.  Have   all  solids   the   same 

weight  ? 

50.  Have  all  liquids  ? 

51.  Define  Specific  Gravity. 

52.  What  makes  oil  float  on  water? 

(Answer:  The  fact  that  oil, 
&c.,  &c.) 

53.  How  does  it  come  that  smoke 

rises,  while  soot  falls? 
(Quest.  9.) 

54.  Why  does  oil  rise  thro' water? 

55.  Why  do  balloons  rise  through 

the  air  ? 
•PAGE  15. — 

56.  Give  law  about  Fluids  of  dif- 

ferent specific  gravity. 

57.  Why  does  a  piece  of  wood 

float,  while  a  st-ne  sinks, 
when  thrown  into  w  iter  ? 

58.  Prove     that    liquids    have 

weight. 

59.  Will  the   weight  of  a  pai1  of 

water  be  increased  when  a 

fish  is  thrown  in  ? 
•60.  Why  does  an  empty  flask  float 

on  water  ? 
61.  Why  does  it  not  also  in  air? 


PAGE  15. — 

62.  Why  does  a  bottle  filled  with 

water  sink  in  water  ? 

63.  Why  does  it  float  on  mercury? 

64.  Under    what     circumstances 

does  a  body  float  ?    sink  ? 

65.  Why  do  iron-clads  float  ? 

66.  When  will  the  body  of  man 

float? 

67.  Why  is  it  difficult  for  bathers 

to  walk  in  water  chin  deep? 

68.  In  drawing  water  from  a  well, 

why  has  the  bucket  more 
weight  as  it  emerges  from 
the  water  ? 

69.  Why  may  heavy  stones  be 

lifted  in  water,  while  on  dry 
land  they  can  scarcely  be 
moved  ? 

70.  What  should  persons  who  can 

not  swim,  do  on  falling  in 
the  water  ? 


71.  Why  does  ice  float  on  water  ? 

72.  Why  does  a  full  tumbler  run 

over  when  a  stone  is  thrown 
in,  and  not  when  a  piece  of 
sponge? 

73.  Why  does  wood  saturated  with 

water,  sink  ? 

74.  ^  'hy  do  some  bodies,  floating 

on  water,  sink  in  it  more 
than  others;  thus  oak  wood 
more  than  pine  wood  ? 

75.  Why   can    persons    float    on 

water  by  means  of  life-pre- 
servers or  bladders  filled 
with  air  ? 


QUESTIONS. 


155 


76.  Why  do  we  often  see  a  sedi- 

ment on  the  bottom  of  ves 
Sels  containing  liquids,  after 
they  have  leen  standing  for  a 
time? 

77.  Why   do     drowned    persons, 

after  having  lain  under  water 
for  a  time,  rise  to  the  sur- 
face ? 

78.  Why  do  ships  sink  deeper  in 

river  water  than  in  the  ocean? 

79.  Why  does  a  hen's  egg  float  on 

water  strongly  salted,  while 
it  sinks  in  fresh  water  ? 


80.  Why  does  water  in  a  vesse' 

rise  higher  on  dropping  intc 
it  a  pound  of  iron  than  i; 
does  when  a  pound  ot  lead 
is  dropped  in  ? 

81.  Why  must  a  dog  sometimes 

drop  a  heavy  stone  (after  hav- 
ing fetched  it  from  the  bot- 
tom of  a  water)  when  he 
reaches  the  surface? 

82.  What  enables  fish  to  move  up 

and  down  in  the  water  at 
pleasure  ? 


LESSON  III.— MAGNETIC  ATTRACTION. 


PAGE  17. — 

83.  Under  what   circumstances 

will    a    plumb-line    change 
from  the  vertical   direction  ? 

84.  Will  it  also  change  if  its  weight 

is  a  stone  ? 

85.  Mention  a  force  which  may 

overcome  gravity. 

86.  Show  that  magnets  and  un- 

magnetic  iron    attract   each 
other. 

87.  Give  a  property   common  to 

both,  magnetic  attraction  an J 
gravity-attraction. 

PAGE  1 8. — 

88.  Where   does  the  power  of  a 

magnet  chiefly  reside  ? 

89.  What  is  the  difference  between 

a  magnet  and  a  piece  of  un- 
magnetic  iron  ? 

90.  What  is  the  name  of  the  ends 

of  the  magnet  ? 

91.  Where  do  these  ends  point  ? 

92.  In   what    position   must   the 

magnet  be  in  that  case  ? 


PAGE  1 8. — 

93.  State  the  law  of  direction  of  a 

magnet. 

94.  What   action   is   seen  in  two 

magnets    whose    like   ends 
are  brought  together  ? 
PAGE  19. — 
95., Give  law  for  it. 

96.  Whence    the     application   of 

magnets  ? 

97.  Is  a  magnetic  needle  liable  to 

deviate  more  on  a  wooden 
vessel  than  on  an  iron  ? 

98.  How  may  a  magnet  be  made  ? 

99.  Why    have    magnets   usually 

that  form  ? 

100.  Describe  a  magnet. 

101.  Does  the  earth  act  like  a  mag-' 

net? 
Give  reasons  for  your  answer. 


102.  What  reason  have  the  French 
for  calling  the  north  pole  of 
a  magnet  its  "South  Pole," 
and  the  south  pole  its  "North 
Pole?" 


LESSON  IV.— ELECTRIC  ATTRACTION. 


PAGE  20. — 

103.  Whence   the   term    "Electri- 

city ?" 

104.  What  power  may  sealing-wax, 

sulphur  and  glass  acquire; 
and  on  what  condition? 


PAGE  20. — 

105.  Same,  regarding  paper. 

1 06.  State  the  source  of  electricity. 

107.  What    peculiar    property    do 

electric  bodies  manifest    . 


156 


FIKST   LESSONS   IN   PHYSICS. 


PAGE  21.— 

108.  What  phenomena  may  accom- 

pany electrified  bodies  ? 

109.  Why   the  peculiar    sensation 

felt   on    holding    electrified 
objects  against  one's  face? 

PAGE  22.— 

110.  Wrhat   becomes  of  electricity 

after  it  has  left  the  sulphur, 
or  the  glass  ? 

in.  Mention  two  good  conductors 
of  electricity. 

112.  Three  non-conductors. 

113.  Give  difference  between  good 

conductors  and  non-conduc- 
ors. 

Can   electricity    be   produced 
upon  both  classes  of  bodies? 

PAGE  23. —  4 

114.  What  phenomena  take  place 

when  electrified  sealing-wax 
is  presented  to  a  suspended 
pith  ball  ? 

115.  When,    only,   do    they    take 

place  ? 

1 1 6.  Did  you  notice  anything  simi- 

lar in  magnets  ? 

117.  In  gravity  ? 

1 1 8.  What  phenomena,  when  elec- 

trified sealing-wax  is  pre- 
sented to  two  pith  balls  ? 

1 19.  What  force  is  o.vercome  in  that 

case? 

1 20.  Was    that    same    force    ever 

overcome  before  ?  (Com p. 
question  85.) 


PAGE  23. — 

121.  What  phenomena,  if  first  seal- 

ing-wax and  then  glass  is 
presented  to  the  single  pith 
ball? 

122.  How   do    you    explain    your 

answer  ? 
PAGE  24. — 

123.  What  phenomena  if  first  seal- 

ing-wax and  then  glass  is 
presented  to  one  of  the  two 
pith  balls  ? 

124.  What  phenomena  if  to  one  of 

the  two  pith  balls  you  pre- 
sent sealing-wax,  and  at  the 
same  time,  glass  to  the  other? 

PAGE  25. — 

125.  How  many  kinds  of  electri- 

city?    Name  them. 

126.  Give  law  of  electricity. 

127.  Explain  principle   and   action 

of  Lightning  Rods. 


128.  On  rubbing  glass  on  flannel, 

do  you  produce  only  one 
kind  of  electricity,  or  both 
kinds  ? 

129.  Why  does   an    electrified  bar 

of  sealing-wax  gradually  lose 
its  electricity  ? 

130.  Why  do  small  pith  balls  upon 

a  table  jump  up  and  down, 
if  a  sheet  of  electrified  paper 
be  held  over  them. 


LESSON  V.— LIGHTNING.— LIGHTNING-RODS. 


PAGE  26. — 

131.  What   was    Franklin's    merit 

regarding  the  explanation  of 
lightning  ? 

132.  Give  an  account  of  Franklin's 

experiment. 

Why    the    pointed    iron 

wire  on  top  of  his  kite  ? 

133.  Could  he   have   taken   a  silk 

string  instead  of  a  hempen 
one  ? 

134.  What  was  the  purpose  of  the 

the  key  ? 


PAGE  27. — 

135.  What  made  the  fibres  of  the 

string  bristle  up  ? 

136.  What  does   Franklin's  experi- 

ment demonstrate  ? 

137.  What  is  the   cause   of  light- 

ning? 

138.  Give  three  paths  which  light- 

ning may  follow  ? 

139.  What  objects  are  most  liable 

to  be  strucl;  I   aid  why  ? 


QUESTIONS. 


157 


PAGE  27. — 

140.  Why   should    you   not   stand 

under   a   tall  tree   during  a 
thunderstorm  ? 
PAGE  28. — 

141.  Which  is  the  safest  place  in  a 
.  room   during  a   thunder- 

stoi  m  ? 


PAGE  28. — 

142.  Give  an  account  of  the  light- 

ning-rod. 

143.  On    what    conditions    may   a 

lightning-rod  be  called  good? 

144.  What   becomes  of  the  light- 

ning   after    passing     down 
'        along  the  rod  ? 


LESSON  VI.— COHESION. 


PAGE  29. — 

145.  Why  is  it  that  meat  must  be 

cut,  while  bread  may  easily 
be  broken  ? 

146.  Why  is   water  easily  divided, 

while  ice  is  not  ? 

147.  What  is  the  name  of  the  force 

which  causes  the  parts  of  a 
solid  to  remain  together  ? 

148.  Why  is  rolled  iron  stronger 

than  common  iron  ? 

149.  To    what     purpose    does    a 

knowledge  of  the  cohesive 
force  serve  ? 

150.  Could  birds  fly  in  water  ? 
PAGE  30. — 

151.  Why  would  it  be  difficult  for 

us  to  walk  through  molasses? 

152.  What  must  be  done  to  break 

a  body  ? 

153.  Why  has  a  walking-cane  lost 

its  strength  if  after  being 
broken,  the  parts  are  glued 
together  ? 


PAGE  3 1.— 

154.  What  is   the  great  enemy  of 

cohesion  ? 
155-  Why    does    oil    form    larger 

drops  than  water  ? 

156.  What  are  pores  ? 

157.  Why  is  a  dry  sponge  smaller 

than  a  wet  one  ? 

158.  What     makes    blotting-paper 

remove  fresh  ink  ? 

159.  Why  do  doors,  window-frames 

and  drawers   often   swell  in 
damp  weather  ? 

1 60.  How  is  :t  that  mercury  can  be 

pressed    through   a  leather 
bag? 

161.  What  causes  wooden  tubs  to 

leak  in  summer  ? 

162.  What  may  be  done  to  prevent 

this  ? 

163.  How   do  solids,  liquids  and 

gases   differ  as  to  cohesion. 

164.  Give  2  app'c  of   cohe.  force. 


LESSON  VII.— ADHESION— CAP.  ATTRACTION. 


PAGE  32.— 

165.  How  can  two  leaden  bullets 

be  made  to  adhere  ? 

1 66.  Why  do  not    two    bricks  ad- 

here in  the  same  manner  ? 

167.  When,    only,    does    adhesion 

take  place  ? 
LV.GE33.— 

1 68.  How  may  two  rough  surfaces 

be  made  to  adhere  ? 

169.  Why  does  the  hand  become 

wet    when     immersed    in 
water?  (Given  in  text.) 

170.  vVhy  does  it  remain  dry  when 

drawn  out  of  mercury  ? 


PAGE  33 — 

171.  What  two  forces  are  in  strug- 

gle with  each  other  when  the 
hand  is  placed  in  water  ? 

172.  Define  adhesion. 

173.  Why  are   two    smoothly  pol- 

ished plates  separated  with 
great  difficulty,  if  laid  to- 
gether and  firmly  pressed  ? 

174.  Why  does  fresh  paint  adhere 

to  one's  dress  ? 
PAGE  35. — 

175.  What  is  a  capillary  tube 

176.  Define  capillary  attraction. 


158 


FIRST   LESSONS   IN   PHYSICS. 


PAGE  35. — 

177.  What   causes   the  sponge  to 

absorb  water?  (Comp.  157.) 

178.  Why   may  eggs  and  meat  be 
kept  fresh  in  sand  ? 

179.  Explain    the  action  of  oil  in 

lamp- wicks. 

1 80.  How,    and  why,  may  grease 

spots  be  removed  from  the 
floor? 


181.  Why  do  two  papers   pasted 

together  adhere  firmly  ? 

182.  Why  may  the  hand  be  drawn 

out  of  the  water  dry,  if,  be- 
fore immersed  it  was  cover'd 
with  Lycopodium  powder? 


183.  Why  is  a  greased  glass  not 

moistened  when  immersed 
in  water  ? 

184.  Why    does   a   small  drop  of 

water  on  a  board  remain  tho' 
the  board  be  inverted? 

185.  Why  does  a  drop  of  mercury 

fall  when  the  board  is  in- 
verted ? 

186.  Why   does   a  small  drop  of 

mercury  on  a  tin  plate  re- 
main when  the  plate  is  in- 
verted ? 

187.  Why  do  figures  drawn   with 

the  finger  upon  a  window- 
pane,  become  visible  if  we 
breathe  on  them  ? 


LESSON  IX.— ELASTICITY. 


PAGE        . — 

1 88.  Why  is  the  spot  which  an 

ivory  ball  receives  upon 
falling  on  a  blackened  sur- 
face, larger  if  the  ball  has 
fallen  from  a  considerable 
height  than  if  it  has  merely 
been  pressed  with  the  hand 
upon  that  surface? 

189.  What   makes  an  arrow,  shot 

from    a    cross-bow,    fly   a 
great  distance  ? 
What    makes     steel,     ivory 
and    india-rubber     resume 


190. 


PAGE  40.— 

their  former  position  after 
being  bent  ? 

191.  Define  elasticity  ?   (El.  is  the 

property  of    bodies  to  re- 
cover their  former  fig.,  etc. 

192.  When  are  bodies  hard — soft  ? 
PAGE  41. — 

193.  Mention   four  applications  of 

the  elasticity  of  bodies. 

194.  Define  btittleness — ductility. 

195.  Define  malleability. 

196.  Give     examples    of     brittle, 

malleable  and  ductile  bod's. 


LESSSON  X.— ELASTICITY  OF  AIR. 


PAGE  42. — 

197.  Show  that  air,  like  every  other 
body,  maintains  its  place. 

PAGE  43. — 

198-  Why  does  not  water  enter  a 
bottle  in  the  neck  of  which 
a  funnel  is  cemented  ? 

199.  Describe   the    action  of  the 

pop-gun.     (Page  44.) 

200.  What  is  its  principle  ? 

201.  Principle  of  the  blow-gun? 

202.  Principle  of  the  Diving-bell  ? 


PAGE  43  — 

203.  W  hat  causes  the  air  inside  a 

Heron's    Fountain    to    be 
compressed  ? 

204.  Desci  ibe  action  of  Heron's  F. 
PAGE  44. — 

205.  Give  the  law  on  elast'y  of  ah. 

206.  What  is  an  air-chamber  ? 

207.  Describe  its  action. 


208.  Why  do  fire- wheels  turn  ? 

209.  Why  do  sky-rockets  ascend  ? 

210.  Why  do  cannons  recoil  when 

fired  off? 


QUESTIONS. 


159 


LESSON  XL— PRESSURE  OF  AIR. 


PAGE  46. — 

211.  Why  does  the  water  not  flow 

from  a  filled  inverted  tum- 
bler, with  a  piece  of  paper 
pressing  against  it  ? 

212.  Show  that  air  presses  down- 

ward. 
PAGE  47. — 

213.  Show  that  air  presses  in  all 

directions. 

214.  Why   does   not  vinegar  flow 

from  a  barrel  whose  bung- 
hole  is  closed  ? 


215.  Explain    the    action    of    the 
"Thief." 


2 1 6.  Why    do    we    not    feel    the 

pressure  of  air  exerted  upon 
us? 

217.  Why  are  travelers  more  easily 

fatigued  on  high  lands  than 
on  low  lands  ? 

2 1 8.  What  makes  us  feel  tired  dur- 

ing excessive  heat,  or  before 
a  thunderstorm  ? 

219.  Wh^  is  it  that,  when  a  bottle 

filled  with  air  in  the  low 
land  is  taken  up  on  high 
land,  the  air  will  escape  with 
violence  when  the  bottle  is 
opened  ? 


LESSON   XIL— BAROMETER. 


PAGE  48. — 

220.  What  can  be  the  height  of  a 

col.  of  water  supported  by 
the  atmosphere? 

221.  What  is  the  height  of  a  col- 

nmn  of  mercury  ? 

222.  Comp.  the  weight,  thickness 

and  height  of  these  liquid 
columns  with  the  corresp'g 
column  of  air. 
PAGE  49. — 

223.  Describe  barometer,  (p.  50. ) 

224.  Why  dues  it  read  the  same 

whether  in  or  out  of  doors? 
22$.  What  are  the  chief  uses  of 

the  barometer? 
PAGE  51. — 

226.  Lxplain  its  use  in  measuring 

heights. 

227.  Its  use   as  an   index  of  the 

weather. 

228.  What   is  the   real   object  of 

the  barometer  ? 

229.  What  causes  the  mercury  in 

the  barometer  to  fall  ? 

230.  What  causes  it  to  rise  ? 

231.  Suppose  we  wished  to  em- 


PAGE  51.—, 

ploy  water  instead  of  mer- 
cury, how  high  would  the 
barometer  tube  have  to  be 
made  ? 

232.  What  is  a  vacuum  ? 

233.  What  is  the  amount  of  the 

pressure  of  air  ? 

234.  How  can  this  be  proved? 


235.  To  what  extent  may  wind  in- 

fluence the  barometer?  (Re- 
member that  wind  is  air  in 
motion. ) 

236.  Why  does  the  mercury  in  the 

barometer  fall  when  carried 
up  on  the  mountains  ? 

237.  Does    atmospheric   pressure 

increase  or  decrease,  as  we 
go  away  from  the  earth  ? 

238.  Supposing  the  moon  to  have 

a  terrestrial  atmosphere, 
how  high  would  the  mer- 
curial column  stand  there  ? 

239.  How  high  on  the  sun  ? 

240.  At  the  centre  of  the  earth  ? 


160 


FIRST   LESSONS   IN   PHYSICS. 


LESSON  XIV— INERTIA. 


PAGE  54, — 

241.  Snow  that  a  body  at  rest  re- 

mains  at   rest   until   set  in 
motion  by  some  force. 

242.  Show  that  for  a  body  !b  be 

set  in  motion,  time  is  neces- 
sary. 
PAGE  55. — 

243.  Show  that  a  body  once  in  mo- 

tion remains  in  motion  until 
stopped. 

Show  that  for  the  motion  of  a 
body  to  stop,  time  is  neces- 
sary. 

244.  Define  inertia. 

245.  Why  is  a  fly-wheel  an  applica- 

tion of  inertia. 


246.  Why  may  the  loose  handle  of 

a  hammer  be  fastened  again 
by  knocking  the  end  of  the 
handle  against  a  hard  object? 

247.  Why  may  a  stopped-up  pipe 

be  cleaned  again  by  forcibly 
knocking  against  one  of  its 
ends  ? 

248.  Why  must  good  bridges  have 

a  great  mass  ? 

249.  Why    may   a'  candle   be  shot 

from  a  great  distance  thro' 
a  board  ? 

250.  Why  do  cannon  or  rifle  balls 

make  a  circular  hole  if  fired 
at  a  window-pane  ?. 

LESSON  XV.— INCLINED  PLANE.         / 

PAGE  58. — 

258  Why  does  a  bullet  thrown 
with  the  hand  inflict  less 
harm  than  one  fired  from  a 


PAGE  56. — 

251.  What  is  an  Inclined  Plane  ? 

252.  Why  does  a  ball  on  it  fall  ? 

253.  Give  three  familiar  instances 

of  an  inclined  plane. 
PAGE  57. — 

254.  Show  that  the  steeper  an  in- 

clined plane  is,  the  greater 
is  the  velocity  of  a  body  fall- 
ing on  it. 

255.  Show   that  in   that  case  the 

force  required  to  ascend  it  is 
greater. 

256.  Show  that  a  body  increases  in 

velocity  as  th>»  space  in- 
creases through  which  it 
falls. 

257.  Show  that  the  greater  the  ve- 

locity of  a  body,  the  greater 
its  striking  force. 


gun  ? 

259.  Why  may    hailstones   destroy 

standing  grain  ? 

260.  What  does  the  falling  of  bodies 

on  an  inclined  plane  show  ? 

261.  Whence  the  practical  applica- 

tion of  the  inclined  plane  ? 

262.  What  is  the  principle  of  the 

wedge  ? 

263.  of  the  ax  ? 

264.  of  the  skid  ? 

265.  Why   are    roads    leading    up 

steep    mountains    made    in 
windings  ? 

266.  What  is  meant  by  the  length 

of  an  inclined  plane  ? 

267.  What,  the  height? 


LESSON  XVI.— LEVER.  \ 

PAGE  59. —  PAGE  59. — 

268.  When  is  a  balanced  rod  in  a     271.  What  is  to  be  noticed  in  lift- 

state  of  equilibrium  ?  ing   the   end  of  the   longer 

269.  Why  then  ?  *rm  writh  the  hand  ? 

272.  What,  if  the    lengths    of  the 

270.  Why  will  the  longer  arm  of  a  two  arms  have  the  ratio  cf 

rod  fall  ?  I  to  2  ? 


QUESTIONS. 


161 


PAGE  59. — 

2 73-  What   characterizes    the    end 
of  the  long  arm  of  a  lever  ? 
PAGE  60. — 

274.  What  have  hailstones  in  com- 

mon with  a  small  weight  at 
the  end  of  the  long  arm  ? 

275.  Define  the  lever. 

276.  Give  a  general  law  about  it. 

277.  What    does    the    amount    of 

power  needed  to  lift  a  load 
by  means  of  a  lever,  depend 
upon? 

278.  How  may  it  be  found  ? 

PAGE  61. — 

279.  Give    the    three    important 

points  in  a  lever. 


PAGE  61. — 

280.  What  is   a  lever  of  the  first 

class  ? 

281.  Give  three  examples,  and  ex- 

plain. 

282.  What  is  a  lever  of  the  second 

class  ? 

283.  Give  three  examples,  and  ex- 

plain. 

284.  To  which  class  of  levers  does 

the  oar  belong  ?  and  why  ? 

285.  The  wheel -barrow  ? 

286.  How   may  a  heavy   stone  be 

lifted  ? 

287.  What  is  the  stone  then  called? 

288.  Why  are  levers  used  only  for 

moving  loads  through  short 
distances  ? 


of  a 


LESSON  XVII.— THE  PENDULUM. 

PAGE  66. — 

297.  What    is    the    office    of   the 

crutch  ? 

298.  Explain    how    it   comes  that 

weight  in  a  clock  causes  the 
hands  to  move  with  uniform 
velocity. 

299.  What  is  the  motory    force   of 

clocks  ? 

300.  What  the  regulating  force  ? 


PAGE  63. — 

189.  What  is  a  vibration? 

290.  Explain    the    vibration 

pendulum. 

291.  How  many  forces  act  upon  it, 

and  what  are  they  ? 

PAGE  64. — 

291.  Show  that  the  vibration  of  the 
same  pendulum,  whether 
quite  short  or  not,  takes 
place  in  the  same  length  of 
time. 

293.  Show  that  a  short  pendulum 

vibrates  more  quickly  than 
a  long  one. 
PAGE  65. — 

294.  What  is  the  principle  applica- 

tion of  the  pendulum  f 

295.  Explain  its  action. 

296.  What  is  meant  by  winding  up 

a  clock  ? 


Would  a  pendulum  placed 
high  up  above  the  earth's 
surface,  vibrate  more  quickly 
or  more  slowly  than  on 
earth? 

How  on  the  moon  ? 

303.  How  on  the  sun? 

304.  Midway  between   the  earth's 

surface  and  center? 


301- 


302. 


305.  At  the  center  of  the  earth  ? 

LESSON  XVm.--COM]Vfl!fNlCATING  VESSELS-HYDRAULIC 
PRESS. 

PAGE  67.—  PAGE  68.— 

306.  Show  that  the  surface  of  quiet    309.  Explain  fountains. 

310.  What  causes  fountain-jets  to  be 

shorter  than  they  ought  to  be? 

311.  How   does   it   come    that  our 

water-pipes    can    lead  water 
to  the  upper  part  of  houses, 


water  is  always  level. 

307.  How   does   water  stand  in  a 

tea-pot  ? 
PAGE  68. — 

308.  Show   that    your    statement 

must  be  true. 
II 


contrary  to  gravity  ? 


162 


FIRST   LESSONS   IN   PHYSICS. 


PAGE  68. — 

312.  Define   Communicating    Ves- 

sels. 

313.  Why  may  water-pipes   under 

ground  be    said  to  be  com- 
municating tubes?     (Text.) 

314.  Give   law   about   pressur 

liquids. 


PAGE  70. — 

315.  Demonstrate  it. 

316.  Give  name  and  date  of  its  ap- 

plication. 

~,     PAGE  70. — 

of    317.  Explain  the  action  of  the  hy 
draulic  press. 


LESSON  XIX.— BREATHING— BELLOWS. 


PAGE  71. — 

318.  Why  can  we,  with  a  tube,  suck 

up  water  with  the  mouth  ? 

319.  Explain  the  process  of  Inspi- 
ration. 

320.  That  of  Expiration. 

321.  What  is  meant  by  breathing? 

322.  What  is  a  vacuum  ? 

323.  What  takes  place  when  air  has 

access  to  a  vacuum? 
PAGE  72. — 

324.  Explain  the  action  of  the  bel- 

lows. 

325.  What  is  a  valve  ? 


PAGE  72. — 

326.  Compare  the  action  of  the  bel- 

lows   with     the    action    oi 
breathing. 

327.  Explain  the  act  of  smoking. 

328.  That  of  drinking. 

329.  Could  we  breathe  in  a  vacu'm? 
Give  reasons  for  your  answer. 


331 


Would  the  bellows  work  in  a 

vacuum  ? 
Give  reasons. 
Would   the  bellows  work  in 

water  ? 


LESSON  XX.— COMMON  PUMP. 


PAGE  74. — 

332.  Explain  the  action  of  the  sy- 
ringe. 

333-  What  causes  the  water  to  rise 
in  it  ? 

334.  Would   it  rise  if  water  and 

syringe,    both,    were    in    a 
vacuum  ? 

335.  What  are  the  principal  parts 

of  a  common  pump  ? 
PAGE  75.— 

336.  Where  is  the  piston  when  the 

handle  is  drawn  out  farthest? 
PAGE  76. — 

337.  When    is    the    piston  at    its 

highest  ? 

338.  In   that   case,   what  is  below 

the  piston  ? 

339'  When  the  piston  commences 
rising,  which  of  the  two 
valves  is  opened  ? 

340.  Why  ? 

341.  When  the  piston  is  at  its  high- 

est, which  valve  is  closed  ? 


PAGE  76. — 

342.  What  is  meant  by  rarified  air? 

343.  Why  does   the  water  in  the 

suction-pipe  rise  ? 

344.  What   is    the   position  of  the 

valves    when   the   piston  is 
being  lowered? 

345.  What  do  you  pump  out  first  ? 
340.  What  causes  the  water  to  flow 

out  through  the  spout  ? 

347.  What  causes  the  lower  valve 

to  close  ? 

348.  What,  the  higher? 

349.  Give  the  principle  of  the  com- 

mon pump. 

350.  What  is  a  pump  ? 

351.  To  what  purpose  is  the  lower 

valve  ? 

352.  The  upper  ? 


353.  Is  there  any  similarity  between 

the  common  pump  and  the 
bellows  ? 

354.  Explain  your  statement. 


QUESTIONS. 


163 


355' 


Since  the  one  serves  to  pump 
out  water,  and  the  other  to 
pump  out  air,  why  has  the 
latter  but  one  valve  ? 


356.  Comparing  the  common  pump 
with  the  barometer,  give 
four  points  which  they  have 
in  common. 

357-  Eight  points  of  difference. 


LESSON  XXL— FORCING   PUMP— FIRE-ENGINE. 


PAGE  77.— 

358.  How  high  (theoretically  speak- 

ing) may  water  be  lifted  with 
a  common  pump? 

359.  Give  reason. 

360.  To  elevate  water  to  a  greater 

height,  what  must  be  used? 
PAGE  78. — 

361.  Give  three  points  in  which  it 

differs   from    the   common 
pump. 

362.  What  are  the  principal  parts 

of  the  forcing  pump? 

363.  Where  is  the  piston  when  the 

handle  is  at  its  lowest? 
When  at  its  highest  ? 

364.  When  the  piston  is  at  its  high- 

est what   is  the  position  of 
the  valves  ? 

365.  When   does  the  lower  valve 

close  ?  and  why  ? 

366.  When  is   the    upper    valve 

opened? 

367.  Why  does  it  close  ?  and  when? 

368.  Are  both  valves  ever  open  at 

the  same  time? 
Closed  at  the  same  time  ? 

369.  Wrhy  not  ? 

370.  When  the  piston   rises,  why 

does  which  valve  open? 


PAGE  78. — 

371.  When  the  piston  is  at  its  low- 

est which  valve  is  open  ? 

372.  Why  does  not  the  water  flow 

back  from  the  tube  ? 

373.  Explain  the  action  of  the  forc- 

ing pump. 

374.  Why  is  it   that,  by  means  of 

this   pump,    water   may   be 
raised   higher    than   by  the 
other  ? 
PAGE  79. — 

375.  Give  the  parts  of    the  Fire- 

Engine. 

376.  Why   does  it  not  have   com- 

mon pumps  ? 

377.  Explain  its  action. 
PAGE  8O. — 

378.  What  causes  the  flow? 

379.  What  makes  the  flow  continu- 

ous? 

380.  Give  the  difference  between  a 

Heron's  fountain  and  an  air- 
chamber  ? 

381.  W7hich  of  the  two  would  work 

better  in  a  vacuum  ? 

382.  How  long  will  either  of  the 

two  "run"? 


LESSON  XXIIL— SOUND. 


PAGES  85  AND  119. — 

383.  What  causes  sound  ? 

384.  What  is  sound? 

385.  What  makes  us  hear  the  crack 

of  a  whip  ? 

386.  Would  we  hear  it  if  there  were 

no  air  ? 

387.  Show  that  sound  is  the  effect 

of  a  vibratoty  motion. 
PAGES  86  AND  120. — 

388.  What  are  sound-waves? 


PAGES  86  AND  120. — 

389.  Do  we  hear  all  sound-waves  ? 

390.  Give  velocity  of  sound. 

391.  What  causes  the   noise  when 

paper  is  torn?    (Text.) 

392.  When  wood  is  broken  ? 

393.  When  a  whip  is  cracked  ? 

394.  Why  do  we  not  hear  the  alarm 

of  a  clock  in    an  exhausted 
receiver  (in  a  vacuum)  ? 


164 


FIRST   LESSONS  IN   PHYSICS. 


395.  Why  is  music  heard  mOre  dis- 

tinctly when  near  than  at  a 
great  distance  ? 

396.  Why  do  some  bodies  give  a 

louder  sourd  than  others? 
(Because  they  have  a  different 
degree  of  elasticity.) 

397.  Why  r,  the  ax  of  a  wood-chop- 

per, at   a   distance,  seen  to 
fall  before  the  blow  is  heard? 

398.  Why    may    distant    cannon- 

thunder,  be  heard  better  by 


putting  the  ear  on  the 
ground  ? 

399.  Why  do  deaf  persons  not  hear? 

400.  Why  are  the  bells  of  a  neigh- 

boring place  heard  ringing 
at  times,  and  not  at  other 
times  ? 

401.  Why  is   it   so  quiet  on    the 

mountains  ? 

402.  Why  need  we  not  speak  so 

loud  on  a  calm  lake  as  on 
land? 


LESSON    XXIV.— EVAPORATION,     FOG,     CLOUDS,    RAIN, 
SNOW,  HAIL,  &c.,  &c. 


PAGE  88.— 

403.  Define  evaporate. 

404.  When  does  evaporation  take 

place  ? 

405.  What  change  does  it  effect? 

406.  Why  is  the  breath  visible  in 

winter? 

407.  Is  all  aqueous  vapor  visible? 
PAGE  89. — 

408.  When  is  it  visible  ? 

409.  What  name  has  it  then  ? 

410.  Under     what     circumstances 

does  it  become  visible  higher 
up  in  the  atmosphere? 

411.  What  name  has  it  then? 

412.  Why    do    clouds   stay  in   the 

air? 

413.  Why  do  soap-bubbles? 

414.  Why  is  it  that  the  higher  up 

the  clouds,  the  greater  the 
rain-drops  ? 

415.  What  is  rain? 
PAGE  90.  — 

416.  What  is  snow? 

417.  What  is  strange  about  hail? 

418.  Whence  the  drops  on  the  out- 

side of  a  tumbler  with  cold 
water,  in  summer  ? 

419.  Why  do  iron  safes  "sweat  "? 

420.  Whence    the   moisture   on    a 

window-pane  when  a  person 
breathes  against  it? 


PAGE  90. — 

421.  When  is  aqueous  vapor  con* 

densed  ? 

422.  What  is  meant  by  condense? 
PAGE  91. — 

423.  What  is  dew  ? 

424.  Why  is  there  no  dew  in  cloudy 

nights  ? 

425.  Why    none     sometimes,    al- 

though the  sky  is  serene? 

426.  What  is  frost? 

427.  Why  do  we  not  have  frost  in 

summer  ? 

428.  Why  does  it  rain  in   moun- 

tainous countries  more  than 
on  low  land? 

429.  Has  the  direction  of  the  water- 

sheds anything  to  do  with 
the  quantity  of  rain? 

430.  When  does  it  ram  more,  in 

daytime  or  at  night  ? 

431.  Give  your  reasons. 

432.  Why  does    it  not   rain  every 

cold  night? 

433.  Is  snow  useful?     Why? 

434.  Upon   what    does    the    solid 

state  of  water,  its  liquid 
state  and  its  gaseous  state 
depend  ? 

435.  Will  it  do  to  compare  the  at- 

mosphere to  a  boiler  ? 

436.  Give  your  reasons. 


QUESTION'S. 


165 


LESSON  XXV.— HEAT— CONDUCTION  OF  HEAT. 


PAGE  92. — 

437.  Whence  the  sparks  which  we 

see  when  flint  and  steel  are 
struck  together? 

438.  When  a  horse  is  galloping? 

439.  What   effect   is   produced  by 

rubbing  a  copper  coin  on 
the  floor? 

440.  WThy  does  not  a  match  ignite 

by  being  rubbed  against 
glass  ? 

441.  Why  does  it  ignite  on  a  brick  ? 

442.  Why  has  he  his  hands  blis- 

tered who  lets  himself  down 
along  a  rope  ? 

443.  Why  does  a  saw  feel  warm 

after  use? 
PAGE  93. — 
/I/14-  How  is  heat  produced? 

445.  What   may    motion  be   con- 

verted into?  (Were  you  to 
ask  "  Into  what  may  motion 
be  converted?"  the  pupil 
would  be  inclined  to  answer 
merely  a  word  or  two. ) 

446.  Why  do  the  hands  get  warm 

on  holding  them  to  a  heated 
stove  ? 

447.  What  sensation  is  felt  on  keep- 

ing the  end  of  a  wire  in  a 
flame?  Why? 

448.  What  is  conduction  of  heat  ? 

449.  Why  is  it  that  that  wire  (Ques- 

tion 447)  may  be  held  longer, 
if  the  end  in  the  hand  is  en- 
veloped in  paper  ? 
PAGE  94. — 

450.  Why  have  teapots  and  solder- 

ing -  irons  usually  wooden 
handles  ?  (Text. ) 

451.  What  class  of  bodies  are  good 

conductors  of  heat? 

452.  What  is  a  good  conductor  of 

heat  ? 

453.  What  is  a  non-conductor  (or 

bad  conductor)  of  heat? 

454.  Mention    six  non-conductors 

of  heat? 


PAGE  94. — 

455.  When  a  wire  and  a  piece  of 

paper  that  have  been  lying 
on  a  heated  stove,  are  touch- 
ed, the  wire  feels  the  warm- 
er. Why? 

456.  Why  do  iron  stoves  heat  well  ? 

457-  Why  may  ice  be  kept  as  well 
in  a  feather  bed  as  in  an  ice- 
chest  ? 

458.  Why    do    mittens    keep    the 

hands  warmer  than  gloves 
with  fingers  ? 

459.  Why  does  snow  melt  more 

readily  on  a  plank  than  on 
a  rock  ? 

460.  Why  are    steam -chests    and 

steam-cylinders  often  cover- 
ed with  wood  ? 

461.  Why  are  the  walls   of  safes 

often  filled  with  fine  ashes  ? 

462.  Why  do  wide  garments  keep 

us  warmer  than  tight  ones  ? 

463.  Why  are  frame  houses  warm- 

er than  stone  ones  ? 
PAGE  95. — 

464.  Whence  the  use  of  good  con- 

ductors of  heat  ? 

465.  Why  are  metallic  vessels  used 

for  boiling  water  and  other 
liquids  ? 

466.  Whence  the  use  of  bad  con- 

ductors of  heat? 

467.  Give  their    effect  upon  warm 

and  cold  bodies  ? 

468.  How    should   a   tumbler    be 

heated?     Why? 

469.  Why  is  less  heat  given  out  by 

a  stove  when  its  inner  sur- 
face is  covered  with  soot  ? 

470.  What    advantage    in   a   long 

stove  pipe  ? 

471.  Why  is  a  glowing  coal  rapidly 

extinguished  if  placed  on 
iron? 

472.  Why  do  double  windows  keep 

the  room  warm  ? 


166 


FIRST   LESSONS   IN   PHYSICS 


473.  Why  does  cold  wind  chill  us  477.  Give    reason   for   your    state- 

all  through  ?  ment  ? 

474.  Why  does  fruit  ripen  quicker  4;8>  m     does  drawi       the  cur. 

against   a   dark    waif   than  t/ns   down    ^   a   room 

when  isolated  ?^  warmer? 

475-  What  advantage  in  air  being  ,TT1       , 

a  bad  conductor?  479-  Why  does  snow  protect  the 

476.  Does  fanning  us  make  the  air  ground  from  freezing  ? 

around  us  cool? 

LESSON  XXVI.— DRAUGHT. 


PAGE  96. — 

480.  Why   will  paper   strips  held 

over  a  heated  stove,  move 
upward  ? 

481.  Why  will  they  rise  if  let  go? 

482.  What  is  the  universal  effect  of 

heat  upon  bodies  ? 

483.  Why  does  heated  air  rise? 

484.  Explain    the    revolving   of   a 

spiral  paper  owing  to  heat. 

485.  Prove  that  the  air  is  warmer 

near  the  ceiling. 

486.  Why  do  balloons,  smoke  and 

steam  rise?     (Text.) 
PAGE  97.— 

487.  When  is  a  flame,  held  in  the 

upper   opening  of  a   room, 
blown  from  the  room  ? 

488.  How  about  a  flame  held  in  the 

lower  opening  ? 

489.  Give  reasons  for   your  state- 

ments. 


PAGE  97. — 

490.  What  is  draught  ? 

491.  What  is  the  cause  of  draught? 

492.  Why  is  a  lamp   extinguished 

if   the   draught  is    stopped 
below  ? 

493.  Why,  if  its  chimney  is  closed 

above  ? 

494.  Compare  this  with  Experiment 

26,  p.  43. 

495.  Show  that  heated  air  rises,  and 

that  colder  flows  toward  the 
source  of  heat. 
596.  What  is  the  cause  of  winds  ? 

497.  How  long  does  wind  last? 

498.  What  is  ventilation  ? 

499.  Is  it  sufficient  for  the  ventila- 

tion   of  a   room   to   simply 
admit  fresh  air  ? 

500.  Prove    your    statement   by  a 

previous  experiment. 


LESSON  XXVII.— EXPANSION  BY  HEAT-THERMOMETER. 


PAGE  99. — 

501.  Why  does  boiling  water  often 

run  over  ? 

502.  Why  does  a  cold  tumbler  crack 

if  placed  on  a  heated  surface? 

503.  How    may    the    cracking   be 

prevented  ? 

504.  What  is  the  effect  of  heat  upon 

all  bodies  ? 

505.  Why  are  rails  placed  on  the 

track  with   space  between? 

506.  How    are    tires    placed    on 

wheels? 

507.  Why  does  pop-corn  pop  ? 


PAGE  100. — 

508.  When  is  a  substance  said  to 

cool?  JDefine  temperature. 

509.  Give   the    parts  of  the   ther- 

mometer. 

510.  Why  the  vacuum? 

511.  Why  could  not  the  glass  tube 

be  open  above  ? 

512.  Can    you    heat   the   vacuum? 

and  what   will  be  the  effect 
upon  -the  mercury  ? 

513.  Why    does    the    mercury   ex- 

pand from  heat  ? 

514.  Why  does  the  mercury  rise? 
Why  does  it  fall  ? 


QUESTIONS. 


167 


PAGE  101 

515.  How  are  thermometers  made  ? 

516.  How    are    the    freezing    and 

boiling  points  obtained? 

517.  What    advantage   in   dividing 

the  space  between  those  two 
points  into  degrees? 


PAGE  100. — 

518.  Why  are  the  plates  of  metallic 

roofing  not  nailed  together  ? 

519.  When,  and  why,  will  hot  water 

crack  a  cold  tumbler  ? 

520.  What  advantage  in  thermom- 

eters? 


LESSON  XXVIII.— THERMOMETER  COMPARED  WITH 
BAROMETER. 


522 
523 


PAGE   102. — 

521.  How  is  the  blood-heat  point  of 
the  thermometer  obtained? 

How  is  it  marked? 

How  did  Fahrenheit  divide 
the  space  between  the  freez- 
ing and  boiling  points? 

524.  Where  did  he  not  commence? 

525.  Where  did  he  commence? 
PAGE  103. — 

526.  How  did  Reaumur  divide  that 

space  ? 

527.  How,  Celsius? 

528.  WThat  are  the  equivalents  of 

80°  C? 

529.  What  of  50°  C? 

530.  What  of  77°  F? 

531.  What  of  32°  F? 

532.  What  of  17  7-9°  C? 

533.  What  of  40°  C  ? 


PAGE  103. — 

534.  What  is  the  healthiest  tem- 
perature of  a  room  ? 

535-  Where  should  thermometers 
be  placed? 

536.  If  in  New  York  the  mercury 

stands  85,  how  would  it 
stand  in  Paris  (according  to 
C.0)?  (Text.) 

537.  Ho  win  Berlin  (C.0)?  (Text.) 

538.  According    to    those    scales, 

what  numbers  would  indi- 
cate the  blood-heat  point? 
(Text.) 

539.  Indicate  the  point  of  healthiest 

temperature  in  Centigrade 
degrees.  (Text.) 

PAGE  104. — 

540.  Give  four   points    which   the 

thermometer  and  barometer 
have  in  common. 

541.  Give  four  points  they  differ  in. 


LESSON  XXIX.— ATMOSPHERIC  ENGINE. 


PAGE   105. — 

542.  How    is    a    sewing-machine 

made  to  work? 

543.  What  is  rectilinear  motion  ? 

544.  Circular  motion? 

545.  Give  two  instances  of  each  ? 

546.  Who   was  Papin,  and  why  is 

he  celebrated? 

PAGE  1 06. — 

547.  Mention  five  diff.  kinds  of  work 

done  by  the  steam-engine. 
PAGE  107. — 

548.  Describe  Papin's  apparatus. 

549.  Describe  the  experiment  with 

the  same. 

550.  What    causes    the   piston    to 

rise  ? 


PAGE  105. — 

551.  What  to  sink? 

PAGE  1 08. — 

552.  What  is  meant  by  an  atmos- 

pheric steam-engine  ? 

553.  Describe  Savery's  apparatus. 

554.  Compare  it  with  Papin's. 

555.  Give   Newcomen's   improve- 

ments on  Papin's  apparatus. 

PAGE  III. — 

556.  Explain  Newcomen's   atmos- 

pheric steam-engine. 

557.  What  causes  the  piston  in  it 

to  rise?     Explain. 

558.  What  causes  it  to  sink  ?     Ex- 

plain. 


168 


FIRST   LEGSONS   IN   PHYSICS. 


PAGE  III. — 

559.  State  the  principal   points  of 

Papin's  engine.     (Text.) 

560.  Of  Savery's.     (Text.) 

561.  Of  Newcomen's.     (Text.) 


PAGE  III. — 

562.  Compare    Savery's    apparatus 

with  Newcomen's  engine. 

563.  Compare   Fapin's  with  New- 

comen's. 


LESSON  XXX.— STEAM-ENGINE. 


PAGE  112. — 

564.  Who  was  Watt? 

565.  Whence   his    familiarity  with 

the  defects  of  Newcomen's 
engine  ? 

566.  What  was  its  principal  defect  ? 

567.  What  was  the  cause  of  this 

defect? 
PAGE  113. — 

568.  How  great  a  loss  was  caused 

thereby  ? 

569.  What  question  arose? 

570.  What    was    Watt's   first  im 

provement  ? 

571.  Explain. 

572.  What   did    Watt's    next   im- 

provement consist  in  ? 

573.  What  defect  did  it  overcome? 

574.  What  caused  now  the  piston 

to  rise  and  sink  ? 


PAGE  114. — 

575.  Did    henceforth    the    steam 

merely   serve    to  produce  a 
vacuum  ? 

576.  Why   was    Newcomen's    ma- 

chine a  single-acting  engine? 

577.  Why   was     it    used   only   for 

pumping  water  ? 

578.  What    constitutes    a    double- 

acting  steam-engine  ? 

PAGE  115. — 

579.  When  did  Watt  die? 

580.  What  is  meant  by  steam  of  low 

pressure  ?  of  high  pressure  ? 

581.  What  are  high  and  low  press- 

ure engines  ? 

582.  What  is  the  use  of  the  sliding 

valve  ? 
PAPE  1 1 6. — 

583.  Explain  action  of  high-press- 

ure engine. 


LESSON  XXXII.— LIGHT— ITS  SOURCES— DIRECTION. 


PAGE  122. — 

584.  What  are  our  sources  of  light? 

585.  Mention     s  i  x     self-luminous 

bodies. 

586.  What  is  a  self-luminous  body? 

(One  which  makes  its  own 
light.) 

PAGE  123. — 

587.  Are  the  planets  self-luminous? 
Are  they  luminous  ? 

588.  What,  then,  is  the  difference 

between  luminous  and  self- 
luminous  ? 


PAGE  123 

589.  When,   only,   are  bodies  not 

self-luminous,   visible  ? 

590.  Show  that  light  emanates  from 

self-luminous   bodies   in  alt 
directions. 

PAGE  124. — 

591.  Show  that  it  travel?  in  straight 

lines. 
Why    have    opera-glasses 


592. 


straight  tubes  ? 


LESSON  XXXII I. -RADIANT  AND  SPECULAR  REFLECTION. 


PAGE  125. — 

593.  What  makes  our  rooms  light 

in  the  daytime  ? 

594.  How    do   all    objects    reflect 

light  ? 

595.  What  enables  us  to  see  a  pencil? 

596.  A  looking-glass  ? 


PAGE  126. — 

597.  What  is   radiant  reflection  of 

light  ? 

598.  Show  that  there   are   objects 

which   reflect   light  also   in 
certain  directions. 


QUESTIONS. 


PAGE  126. — 

599.  What  is  this  kind  of  reflection 

called? 

PAGE  127. — 

600.  What  class  of  objects   reflect 

light,    both,    radiantly    and 
specularly  ? 


PAGE  127. — 

60 1.  Compare  light  reflected  radi- 
antly with  light  reflected 
specularly  by 

(a.)  Giving  four  points  in  com- 
mon. 

(£.)  Three  points  of  difference. 


LESSON  XXXIV.— VISIBLE  DIRECTION— REFRACTION. 


PAGE  128. — 

602.  Whither   does    a  person    hit 

with  a  stone,  look? 

603.  Show  that  a  boy  looking  at  a 

steeple  does  somet'ng  simiPr. 
PAGE  129. — 

604.  On  page  128  the  two  images 

have  opposite  directions ; 
why  does  the  person,  never- 
theless, see  the  object  in 
only  one  direction  ? 

605.  Give  general  statement  of  the 

case.  (All  bodies  appear  to 
be  situated  &c.,  &c.) 


PAGE  129. — 

606.  Why  do  oars,  when  immersed 

obliquely,  appear  bent  ? 
PAGE  130. — 

607.  Why  does  the  eye  see  the  oar 

bent? 

608.  When   is  a   coin  on  the  bot- 

tom of  a  filled  tumbler  not 
seen   in  its  true  place  and 
direction  ? 
PAGE  131. — 

609.  Why  do  clear  waters  appear 

more  shallow  than  they  are? 

610.  What  is  refraction  of  light? 


LESSON  XXXV.— PRISMS— LENSES. 


PAGE  132. — 

611.  Show  the  passage  of  rays  (of 

an  arrow)  through  a  prism 
with  edge  upward 
PAGE  133.— 

612.  With  the  edge  downward. 

613.  What  is  a  prism  ? 

614.  Show  the  path  of  rays  of  an 

arrow  through"  two  prisms 
with  their  bases  adjacent  (as 
on  page  133). 

615.  Why  the  use  of  a  curved  glass 

in  place  of  the  prisms  ? 
PAGE  134.— 

6 1 6.  What  two  names  are  given  to 

this? 


PAGE  134. — 

617.  What  is  the  Focus  ? 

618.  Why  the  term  Burning-glass  ? 

619.  What  effect   has   such  a  lens 

upon  objects  ? 

620.  Is  your  answer  true  in  every 

sense  of  the  word  ? 
PAGE  135. — 

621.  Give    the   general    statement 

true  of  such  a  lens. 

622.  Does  a  telescope  really  mag- 

nify distant  objects  ? 

623.  Upon  what,  then,  is   its  use 

based  ? 


LESSON  XXXVI.— COLOR. 


PAGE  136. — 

624.  What  is  dispersion  of  light  ? 

6*5.  How  can  it  be  shown  ? 

^6.  What  effect  has  it  upon  white 

light  ? 
627.  Give    the   principal   colors  of 

the  rainbow. 


PAGE  136. — 

628.  What  is  the  whole  series  of 

colors  called  ? 
PAGE  137.-  - 

629.  How  can  you  prove  that  ordi- 

nary sunlight  contains  these 
colors  ? 


170 


FIRST   LESSONS   IN   PHYSICS. 


PAGE  137. — 

'630.  Is  color  a  substance  ? 

PAGE  138. — 

631.  Why   does   white   glass  look 

white  ? 
•632.  Why   does    blue    glass    look 

blue? 

633.  Why  do   objects  near   a  blue 

curtain  have  a  blueish  tinge? 
PAGE  139. — 

634.  When   is   a  body   said  to  be 

colored  ? 
^635.  When  white  ? 


PAGE  139. — 

636.  When  black? 

637.  What    causes  a   piece  of  red 

cloth  to  appear  red  ?  (Text.) 

638.  What,  a  sheet  of  paper  to  ap- 

pear white?  (Text.) 

639.  A  black  coat  to  appear  black? 

(Text.) 

640.  Why  is    everything  black  on 

a  dark  night?     (Text.) 

641.  Is  color  a  quality  inherent  in 

bodies  ? 

642.  Is  it  a  property  of  a  body  ? 


LESSON  XXXVIL— CHEMICAL  ELECTRICITY. 


PAGE  141. — 

'643.  What    constitutes   a   galvanic 
element,  or  cell? 

644.  What  is  a  galvanic  battery  ? 

645.  How  is  a  coke-cell  prepared  ? 
•646.  Why  must   the  cup  used  be 

un  glazed  ? 
(The    two   liquids  pass   each 

other  through  its  pores.) 
•647.  Why  must  the  wire  ends  be 

entirely  clean ! 
(The  electric  current  does  not 

leap  over. ) 


PAGE  141. — 

648.  Whence  the  thrilling  sensation 

if  the  current  passes  through 
the  tongue  ? 
PAGE  142. — 

649.  Explain  the  action  of  such  a 

galvanic  cell. 

650.  How  is  chemical  or  galvanic 

electricity  produced  ? 

651.  Whence  the  rwa\t  galvanic? 

652.  Explain  the  uninterrupted  cur- 

rent of  electricity 

653.  Is  the  length  of  the  wires  of 

importance  ? 


LESSON  XXXVIII.— THE  ELECTRO-MAGNETIC  TELE- 
GRAPH. 


TAGE  144. — 

•654.  What  is  next  to  the  steam- 
engine  the  wonder  of  our 
age,  and  why  ? 

'655.  Who  discovered  the  effect  of 
the  galvanic  current  upon 
iron? 

656.  Who  put  up  the  first  telegr'ph? 
PAGE  145. — 

657.  Who  perfected  the  telegraph? 

658.  How    may    the    principle   of 

Morse's  telegraph  be  illus- 
trated ? 
PAGE  146. — 

659.  What  renders  the  horse-shoe 

rod  magnetic? 

>66o.  Describe  the  path  of  the  elec- 
tric current  of  the  cell,  when 
passing  around  the  rod. 


PAGE  147. — 

661.  What  effect  has  the  interrup- 

tion of  the  current  upon  the 
rod? 

662.  What  is  an  Electro- Magnet 

663.  What  is  the  keeper  ? 

PAGE  148. 

664.  What  are  the  principles  of  the 


re  the  principl 
ic  telegraph  ? 


electric 

665.  Describe    the    apparatus    by 

which  they  may  be  demon- 
strated. 

666.  Compare  the  Magnet  with  the 

Electro-Magnet,  giving 
(a.)  Five  points  in  common. 
(£.)  Two  points  of  difference. 


APPEN  DIX. 


I.— REMARKS. 

LESSON  III. — To  preserve  their  magnetism,  the  poles  of  magnets 
should  constantly  be  kept  in  contact  with  iron. 

LESSON  IV. — Bar-sulphur,  or  a  solid  glass  rod,  is  often  preferred  to  a 
lamp-chimney.  For  negative  electricity,  use  "hard"  rubber  (gutta  percha). 

LESSON  V. — May  be  used  as  a  reading  lesson,  or  as  one  in  which  the 
unfinished  part  of  any  previous  lesson  can  be  brought  up. 

LESSON  VII. — Time  is  gained  if  the  bullets  are  flattened  with  a 
hammer  ;  then  scrape  one  of  the  flattened  surfaces  of  each  with  a  knife ; 
press  the  two  surfaces  together  by  a  few  strokes  of  a  hammer. 

The  glass  plates,  and  tubes,  must  be  free  from  grease. 

The  pores  of  a  body  may  be  compared  to  the  open  ends  of  capillary 
tubes  ;  the  capillary  tubes  may  be  compared  to  glass  tubes  of  very  fine 
bore. 

LESSON  IX. — Instead  of  an  ivory  ball,  which  is  expensive,  a  large 
marble  gives  the  same  result.  The  round  spot  may  be  shown  also  on 
the  slab. 

LESSON  X. — A  piece  of  India-rubber  tubing  around  the  tube  of  the 
funnel  will  do  as  well  as  sealing-wax,  or  any  other  cement.  Any  kind 
of  tube,  about  ^  inch  in  diameter  and  about  3  feet  long,  may  serve  as  a 
blow-gun. 

LESSON  XL — A  tumbler  with  a  brim  curved  outward  is  best. 

LESSON  XVI. — A  square  wooden  beam  about  20  inches  long  for  a 
lever,  with  the  sharp  edge  of  a  ruler,  paper  knife  or  knife-blade  as  a 
fulcrum.  The  fulcrum  must  be  firmly  fixed. 

LESSON  XVIII. — The  bees-wax  must  be  put  on  very  thin,  or  else 
the  water  cannot  force  its  way  through.  Draw  Fig.  1 7  on  the  board. 
The  small  triangular  valve,  when  lowered,  is  just  large  enough  to  close 
the  right  hand  side  of  the  short  horizontal  tube. 

LESSONS  XX  AND  XXI. — Draw  the  Fig.  on  the  board. 

LESSON  XXV. — Copper  being  a  better  conductor  than  iron,  copper 
wire  is  preferable. 

LESSONS  XXIX  AND  XXX. — Draw  the  Fig.  on  the  board. 

LESSON  XXXV — EXPERIMENT  65. — Draw  an  arrow  on  the  board, 
place  the  prism  in  proper  position,  and  sufficiently  elevated  for  each 
scholar  to  see  the  arrow  through  the  prism.  As  this  requires  but  a  few 
seconds,  it  may  be  found  convenient  to  let  the  class  slowly  file  past  the 
prism. 


172  FIRST   LESSONS   IN   PHYSICS. 

II.— GLASS  AND  CORK  WORKING. 

The  following  is  taken  nearly  literally  from  Vernon  Harcourt's  excel- 
lent work,  "Exercises  in  Practical  Chemistry,  Clarendon  Press, 
Oxford:"  . 

1.  To  CUT  A  GLASS  TUBE. — Take  glass  tubing  about  f£-inch  external 
diameter  ("hard  glass  "  is  preferable).     For  a  Hero's  fountain  (Lesson 
X,  Experiment  27,  and  also  frontispiece),  it  should  reach  from  within  half 
an  inch  of  the  bottom  of  the  flask  to  about  eight  inches  above  the  cork. 
As  mercantile  tubing  is  much  longer,  a  piece  of  that  length  should  be 
cut  off.     To  do  this,  lay  the  tube  on  a  table,  hold  it  between  the  thumb 
and  forefinger  of  the  left  hand,  placed  close  to  where  it  is  to  be  cut. 
Take  a  triangular  file,  press  your  left  thumb  and  forefinger  firmly  against 
the  tube,  put  the  edge  of  the  file  upon  the  tube  so  as  to  touch  and  lean 
against  the  thumb,  which  will  thus   prevent  the  file  from  slipping  over 
the   glass,  and  make  a  notch  on   the  glass  by  a  few  short,  energetic 
strokes  in  a  forward  direction.      While  in  the  act  of  cutting,  do  not  bear 
down  too  heavily  with  the  left  hand;  rather  have  your  left  thumb  yield 
a  little  as  the  file  passes   forward,  so  that  the  tube  may  turn  a  little  in 
the  direction  of  the  advancing  file.     To  guard  against  injury,  in  case  the 
tube  should  yield,  put  on  a  glove.      Now  take  up  the  tube,  holding  it  so 
that  the  thumb-nails  are  opposite  to  each  other,  with  the  notch  between 
them,  and  that  you  tightly   press   the   tube  (where  the  notch  is)  with 
thumb-nail  and  forefinger  of  each  hand,  and  with  a  resolute  grasp  break 
the  tube  (moving  your  hands  in  a  direction  from  you)  as  you  would 
break  a  stick.     The  edges  of  the  new  end  will  be  sharp  and  rugged;  to 
prevent  their  tearing  the  cork,  pass  the  file  lightly  over  them;  then  hold 
them  a  few  seconds  in  the  tip  of  an  alcohol  (or  gas)  flame. 

For  a  Hero's  fountain  like  the  one  in  the  frontispiece — a  more  con- 
venient form  than  that  in  Lesson  X — you  should  have  a  bent  glass  tube. 

2.  To  BEND  A  GLASS  TUBE. — If  the  external  diameter  of  the  tube 
does  not  exceed  half  an  inch,  a  common  gas  flame  is  very  suitable;  but 
if  the  gas  is  not  at  hand,  a  spirit  lamp  with  a  large  flame  may  be  used. 
Light  the  gas,  or  spirit  lamp ;   then  holding  the  piece  of  tube  by  its  ex- 
tremities, bring  it  a  little  above  the  flame,  turning  it  constantly  around 
and  moving  it  laterally  so  as  to  heat  about  two  inches  of  it  equally  on 
both  sides.     After  a  few  seconds  lower  it  gradually  into  the  flame,  still 
constantly  turning  it  round. 

If  the  gas  burner  be  used,  the  glass  will  become  covered  with  soot 
when  immersed  in  the  flame ;  but  this  is  of  no  consequence,  as  the  heat 
of  such  a  burner  is  never  high  enough  to  incorporate  the  carbon  with 
the  glass.  When  the  heated  portion  becomes  soft  and  yielding,  which 
will  take  place  even  before  it  has  acquired  a  visible  red  heat,  withdraw 
it  from  the  flame,  and  gently  bend  it  to  a  right  angle,  avoiding  the  use 
of  much  force.  When  the  proper  bend  is  completed,  lay  the  tube  on  a 
bit  of  glass  in  such  a  position  that  the  heated  portion  does  not  come  into 
contact  with  any  cold  surface,  and  leave  it  to  cool  slowly. 

3.  To  MAKE  A  GLASS  JET. — Take  the  straight  tube,  previously  ob- 
tained;   heat  the  tube  two  inches  from  one  of  its  ends  by  holding  it  to 


GLASS   AND   CORK   WORKING  173 

the  extent  oi  half  an  inch  in  the  upper  part  of  a  flame.  The  thumb  and 
forefinger  of  each  hand  should  hold  the  glass  about  an  inch  from  the 
heated  part.  The  heated  part  will  soon  become  soft  and  a  little  narrower. 
Then  withdraw  it  from  the  flame,  and  draw  the  heated  part  out  by  pul- 
ling the  two  ends  of  the  glass  apart.  But  pull  very  gently  or  else  the 
tube  will  be  drawn  out  too  thin;  the  jet  should  have  about  ^-inch 
external  diameter.  Gently  place  the  whole  on  the  table  before  you  and 
allow  it  to  cool ;  then  make  a  fine  notch  at  the  middle  of  the  drawn-out 
part,  and  break  the  tube  there.  The  long  part  is  the  jet  for  a  Hero's 
fountain.  If  the  aperture  is  too  wide,  hold  it  for  a  second  or  two  in  the 
flame. 

4.  To  PERFORATE  A  CORK.— It  now  remains  to  fit  these  tubes — the 
bent  tube  and  the  jet — to  the  bottle  by  means  of  a  cork  having  two  holes. 
Take  a  good,  sound  cork,  about  an  inch  in  diameter,  squeeze  it  until  it 
becomes  soft  and  elastic  (a  pair  of  pliers  or  nut-crackers  will  serve  the 
purpose  of  a  regular  cork-squeezer),  then  take  it  up  between  the  second 
finger  and  the  thumb  of  the  left  hand,  and  place  the  sharpened  end  of 
the  smallest  cork-borer  against  it,  one  end  of  the  cork  midway  between 
the  center  and  the  edge.  Urge  the  cork-borer  into  the  cork  with  a  twist- 
ing motion,  as  if  you  were  using  a  cork  screw.  Some  care  will  be  re- 
quired to  make  the  hole  straight  through  the  cork,  so  that  it  may  be 
truly  central.  Of  the  proper  direction  the  eye  will  be  the  best  judge. 
And  when  the  cork-borer  has  penetrated  some  little  way,  it  will  be  advis- 
able to  turn  the  cork  a  quarter  round  in  order  that  it  may  be  seen 
whether  the  axis  of  the  cork-borer  and  of  the  cork  are  still  in  the  same 
straight  line.  If  not,  a  slight  pressure  on  the  cork-borer  in  one  direc- 
tion or  the  other  will  set  it  straight.  When  the  borer  has  penetrated 
quite  through  the  cork,  it  may  be  withdrawn  with  a  twitching  motion, 
and  will  bring  with  it  a  cylindrical  plug  of  cork,  leaving  a  hole,  the  sides 
of  which  should  be  smoothed  with  a  round  file.  In  the  same  manner 
make  the  other  hole  midway  between  the  center  and  the  circumference. 
Take  a  cork-borer  rather  smaller  than  the  tubing  which  you  have;  see 
that  the  holes  do  not  run  into  each  other,  or  pierce  the  side  of  the  cork. 
The  holes  should  next  be  smoothed  and  slightly  enlarged  by  a  rat-tail 
file,  until  the  end  of  one  of  the  tubes  will  just  enter  them  when  some 
little  pressure  is  used.  (If  much  pressure  is  used,  the  tube  is  not  un- 
likely to  break,  and  the  splinters  of  glass  may  cause  injury.  The  hole 
should  never  be  so  much  smaller  than  the  tube  as  to  make  it  necessary 
to  use  much  force  in  passing  the  latter  through  it.  It  is  a  good  plan, 
also,  to  wrap  the  tube  in  a  cloth  or  handkerchief  while  it  is  being  inserted 
in  the  cork. )  Now  pass  the  longer  of  the  two  tubes  through  the  cork, 
with  moderate  pressure  and  a  twisting  motion,  until  it  projects  so  far  as 
to  reach,  when  the  cork  is  fitted  into  its  place,  nearly  to  the  bottom  of 
the  bottle.  When  this  is  done  pass  the  other  tube  through  the  other 
hole  in  the  cork,  until  it  projects  one  or  two  inches  on  the  other  side. 


174  FIKST   LESSONS   IN   PHYSICS. 

III.— PROBLEMS  ON  THE  THERMOMETER. 
F.  C. 


212. 


32- 


100 Boiling  Ft.     Tne  questions  on  page   103  need  a  few 

explanations.  Only  the  F  and  C  scales 
require  problems,  they  being  the  most 
important.  To  illustrate  the  problems  be- 
low, let  the  two  vertical  lines  annexed 
0 Freez'g  Pt.  represent  the  two  thermometers. 


Problem,  i. — The  mercury  stands  at  86°  F,  how  will  it  stand  accord- 
ing to  degrees  C  ? 

Solution  :  86°  F  =  86  —  32  —  54°  F  above  freezing-point.  Now, 
since  180°  F  above  freezing-point  =  100°  C,  we  have  9°  F  — -  5°  C, 
or  i°  F  =  5  C;  and 54°  =  54  X  f  =  3°°  C  =  answer. 

Rule  I.  —  To  convert  any  number  of  degrees  F  above  freezing-point 
into  degrees  C,  first  subtract  32  from  that  number,  then  multiply  the  re- 
mainder by  5-9. 

Problem  2. — Mercury  at  25°  C,  how  must  this  be  read  in  degrees  F  ? 

Solution  :  100°  C  =  equals  180°  F  above  freezing-point,  or  5°  C 
=  9?  F ;  hence  i°  C  =  |  F,  and  25°  C  =  25  X  f  —  45°  F  above 
freezing-point.  Now,  45°  F  above  freezing-point  means  45°  above  329 
F,  so  we  must  add  45  to  32,  which  is  77°  F,  answer. 

Rule  II. — To  convert  any  number  of  degrees  £,  above  the  freezing-point 
into  degrees  F,  multiply  the  number  by  |,  and  then  add  32. 

Problem.  3. — Thermometer  at  23°  F,  how  is  this  read  in  degrees  C? 

Solution  :  Consulting  the  two  thermometers  represented  above,  we 
find  that  23°  F  means  32  —  23  =  9°  F  below  the  freezing  point.  The 
problem  now  is :  Resolve  9°  F  below  freezing-point  into  its  equivalent 
degrees  C.  Hence  we  have  9  X  |  —  ^  below  freezing-point  =  —  5° 
C,  answer. 

Problem.  4. — Thermometer  at  —  13°  F,  required  its  standing  in  de- 
grees C. 

Solution:  By  inspection  we  find  that  the  0  point  of  the  F  scale  is  32° 
below  the  freezing-point,  and  that  —  13°  is  13°  below  the  0  point,  that  is, 
32  -f  13  =  45°  F.  the  actual  number  of  degrees  F.  below  the  freezing- 
point.  Now,  45^  F  below  freezing-point  X  §  =  25Q  c  below  freez- 
ing-point, or  —  25^  C,  answer. 

Rule  III. — To  convert  any  given  number  of  degrees  F  below  freezing- 
point  into  C,  ascertain  the  difference  between  32°  F  and  the  number 
given,  and  multiply  this  difference  by  |,  the  product  will  be  —  C°. 

Problem.  5. — Thermometer  at  —  30°  C ;  give  the  same  in  degrees  F. 

Solution:  —  30°  C  means  below  the  freezing  point ;  hence  —  30°  C  X 
9  =  54°  F  below  freezing-point.  By  inspection  it  will  be  found  that 
we  must  subtract  54Q  from  32°  ;  or,  32  —  54  =  —  22Q  F,  answer. 

Rule  IV. — To  convert  degrees  C  below  the  freezing-point  into  degrees 
F,  multiply  the  given  number  of  —  C^  by  J ;  subtract  the  product  from 
32°,  and  the  remainder  =  answer.  If  the  subtrahend  is  greater  than 
the  minuend,  as  in  the  problem  above,  the  difference  between  the  product 
and  32*  is  negative,  that  is,  below  0?  F,  and  consequently  marked  —  F° 


INDEX. 


PAGE. 

Academy  of  Florence 31 

Adhesion 32 

Attraction,  Capillary 35 

Attraction,  Electric 2C 

Attraction,  Magnetic 17 

Balance 13 

Barometer 49 

Barometer  comp.  with  Pump.  80 
Barometer   compared    -with 

Thermometer 104 

Bellows 72 

Blotting-paper 38 

Blow-pipe 43 

Breathing 71 

Burning-glass 134 

Cell,  Galvanic 141 

Clock  Weights 13 

Clocks....:.. 65 

Clouds 89 

Cohesion 29 

Color 136 

Compass 38 

Communicating  Vessels 87 

Condenser 113 

Conductors  of  Electricity 22 

Conductors  of  Heat 94 

Contraction  by  Cold 100 

Conversion  of  Force,  Motion,  121 

Contents,  Table  of 7 

Current,  Electric 142 

Dew gi 

Direction,  Visible 128 

Diving-bell 44 

Draught .......  96 

Drowning ^ 16 

Ductile... 


41 

Ductility 29 


PAGE. 

Elasticity .  39. 

Elasticity,  Application  of 41 

Elasticity  of  Air 42 

Electricity,  Pos.  and  N eg 25 

Electricity,  Chemical 140 

Electric  Attraction 20 

Electric  Repulsion 23 

Electro- Magnet 150- 

Element,  Galvanic 141 

Evaporation 88 

Exhalation 71 

Expansion  by  Heat 99 

Fire-Engine 7& 

Fly-Wheels 55 

Fog 89 

Force,  into  Motion 84 

Franklin's  Experiment 26 

Frost 91 

Fulcrum 6l 

Glass  for  Electric  Purposes. . .  24. 

Glass  for  Prism 136- 

Gravity,  Direction  of 1 1 

Gravity,  Force  of 9' 

Gravity,  Specific 14 

Hail 90 

Heat. 92 

Heat,  Conduction  of. 93 

Heron's  Fountain 44 

High  Pressure 1*5 

Horizontal *3 

Hour-glass *3 

Hydraulic  Press 67 

Impenetrability 3«* 

Inclined  Plane 5& 

Inertia 53 

Inhalation 71 


INDEX. 


PAGE. 

Lenses 133 

Level 12 

Lever 59 

Light,  Direction 124 

Light,  Sources 122 

Light,  Radiant  and  Specular 

Reflection 125 

Light,  Radiant  and  Specular 

Reflection  Compared 127 

Lightning 26 

Lightning- Rod 27  38 

Locomotive 117  i  *8 

Low  Pressure 115 

Magnetic  Attraction 17 

Magnet 18 

Magnet  compared  with  Elec- 
tro-Magnet   150 

Malleable 41 

Metals,  Conductors  of  Heat..  94 

Morse's  Telegraph 145 

Needle,  How  rend.  Magnetic.     19 

Newcomen's  Engine 108 

NuJt-Cracker   — ^ 6" 


Papin's  Apparatus 105 

Pendulum 63 

Persons  Drowning 16 

Pith-balls,  How  made 22 

Plumb-line 1 1 

Poles  of  Magnets 19 

Pop-gun 43 

Pores . 31 

Pressure  of  Air   46     50 

Pressure,  Downward 12 

Prisms 132,   136,   137 


PAGE. 

Pump,  Common 74 

Pump,  Forcing 77 

Pull 84 

Push..  8/L 


Radiation  of  Ligfrt 

Rain 

Reflection  of  Light 

Refraction  of  Light 

Refraction  of  Light,  Law . . . 
Repulsion,  Electric 


Self-luminous 

Sliding-valve 

Snow 

Sound 

Spark,  Electric 

Steam-Engine,  Atmospheric. 
Steam-Engine,  Newcomen's. 

Steam-Engine,   Papin's 

Steam-Engine,   Savery's 

Steam-Engine,  Watt's , 


125 
89 

125 
129 

131 

23 

123 

"5 
9° 
85 

21 


I05 
108 

112 


Telegraph •'. 144 

Telegraph,  Principle  of, 147 

Telegraph,   Prin.   Demonst'd.  148 

Thermometer —  ..100,  102 

Thermometer  compared  with 

Barometer 104 

Thermometer  Problems 174 

Vacuum - 49 

Vertical 1 1 

Visible  Direction 128 

Watt,  James...* 112 

Weight 12 

Winds,  Cause  of —  •  98 

Work  done  by  forces 83 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


rtTft 

&«&** 

REC'D    -0 

\ 

NQV    £  BGB 

SENT  ON  ILL 

NOV  0  4  1994 

U.  C.  BERKELEY 

LD  2lA-50m-4,'60 
(A9562slO)476B 


General  Library 

University  of  California 

Berkeley 


!ti   16963 


M81995 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


